Multi-phase electrolyte lithium batteries

ABSTRACT

Electrode assemblies for use in electrochemical cells are provided. The negative electrode assembly includes negative electrode active material and an electrolyte chosen specifically for its useful properties in the negative electrode. Such properties include reductive stability and ability to accommodate expansion and contraction of the negative electrode active material. Similarly, the positive electrode assembly includes positive electrode active material and an electrolyte chosen specifically for its useful properties in the positive electrode. These properties include oxidative stability and the ability to prevent dissolution of transition metals used in the positive electrode active material. A third electrolyte can be used as separator between the negative electrode and the positive electrode. A cell is constructed with a cathode that includes a fluorinated electrolyte which does not penetrate into the solid-state polymer electrolyte separator between it and the lithium-based anode. Such an assembly improves charge transport properties without compromising the strength and durability of the separator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 13/128,232, filed Jul. 22, 2011, now U.S. Pat. No. 9,136,562which, in turn, claims priority to U.S. Provisional Patent Application61/028,443, filed Feb. 13, 2008, to U.S. Provisional Patent Application61/046,685, filed Apr. 21, 2008, to U.S. Provisional Patent Application61/112,605, filed Nov. 7, 2008, and to International Patent ApplicationPCT/US09/063643, filed Nov. 6, 2009, all of which are incorporated byreference herein. This application also claims priority to U.S.Provisional Patent Application 62/160,375, filed May 12, 2015, to U.S.Provisional Patent Application 62/119,107, filed Feb. 21, 2015, to U.S.Provisional Patent Application 62/143,011, filed Apr. 3, 2015, to U.S.Provisional Patent Application 62/144,287, filed Apr. 7, 2015, and toU.S. Provisional Patent Application 62/173,336, filed Jun. 9, 2015, allof which are incorporated by reference herein.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to lithium batteries, and, morespecifically, to the use of a variety of electrolytes in the samelithium battery to optimize its performance.

In order to be useful in a cell, an electrolyte is chemicallycompatible/stable with both the anode material and the cathode material.In addition, the electrolyte is electrochemically stable, that is,stable against reduction at the anode and oxidation at the cathode whenthe cell is at potential. These requirements are especially difficult tomeet in lithium batteries because of the extreme reactivity of thelithium itself. When a liquid electrolyte is used, it permeates both theanode and the cathode, as well as the separator, so the one electrolytemust meet all criteria. Thus some compromises must be made in choice ofelectrolyte, as the electrolyte that is best for the anode and theelectrolyte that is best for the cathode may not be the same.

Thus there is a clear need for a battery cell design in which differentportions of the cell can contain different electrolytes, each optimizedfor its particular function, but all functioning together withoutcompromising the overall operation of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIG. 1 is a schematic illustration of various negative electrodeassemblies, according to embodiments of the invention.

FIG. 2 is a schematic illustration of various positive electrodeassemblies, according to embodiments of the invention.

FIG. 3 is a schematic illustration of an electrochemical cell, accordingto an embodiment of the invention.

FIG. 4 is a schematic illustration of an electrochemical cell, accordingto another embodiment of the invention.

FIG. 5A is a simplified illustration of an exemplary diblock polymermolecule.

FIG. 5B is a simplified illustration of multiple diblock polymermolecules as shown in FIG. 5A arranged to form a domain structure

FIG. 5C is a simplified illustration of multiple domain structures asshown in FIG. 5B arranged to form multiple repeat domains, therebyforming a continuous nanostructured block copolymer material.

FIG. 6A is a simplified illustration of an exemplary triblock polymermolecule, wherein two blocks are the same.

FIG. 6B is a simplified illustration of multiple triblock polymermolecules as shown in FIG. 6A arranged to form a domain structure

FIG. 6C is a simplified illustration of multiple domain structures asshown in FIG. 6B arranged to form multiple repeat domains, therebyforming a continuous nanostructured block copolymer material.

FIG. 7A is a simplified illustration of an exemplary triblock polymermolecule, wherein no two blocks are the same.

FIG. 7B is a simplified illustration of multiple triblock polymermolecules as shown in FIG. 7A arranged to form a domain structure

FIG. 7C is a simplified illustration of multiple domain structures asshown in FIG. 7B arranged to form multiple repeat domains, therebyforming a continuous nanostructured block copolymer material.

FIG. 8 is a differential scanning calorimetry (DSC) trace of thecompound shown as (11) below with a 2.3 mol % pefluro alkane (PFA)repeat unit.

FIG. 9 shows DSC traces for 1) a PEO-graft-PFA copolymer with a 2.3 mol% PFA repeat unit mixed with LiTFSI and an ionic liquid and 2) a PEOhomopolymer mixed with the same amounts of LiTFSI and the ionic liquid.

FIG. 10 shows graphs of ionic conductivity as a function of inversetemperature for cells made with various electrolyte mixtures, accordingto an embodiment of the invention.

FIG. 11 shows a complex impedance plot for a single electrolyte system(x) and for a two electrolyte system (∘).

FIG. 12A shows specific capacity data over 500 cycles for a cell thatcontains one dry polymer electrolyte.

FIG. 12B shows specific capacity data over 100 cycles for a cell thatcontains two dry polymer electrolytes.

SUMMARY

An electrochemical cell is provided. In one embodiment of the invention,the cell has a negative electrode configured to absorb and releasealkali metal ions, a first block copolymer electrolyte separator layerthat includes a first salt that contains the alkali metal, and apositive electrode that has positive electrode active material, binder,optional electronically conducting particles, a fluorinated catholyte,and a second salt that includes the alkali metal. The fluorinatedcatholyte is immiscible with the first block copolymer electrolyte. Theseparator layer is disposed between the negative electrode and thepositive electrode and facilitates ionic communication therebetween. Thefirst salt and the second may or may not be the same.

The separator electrolyte may be a solid electrolyte. The separatorelectrolyte may include any of ceramic electrolytes, polymerelectrolytes, and block copolymer electrolytes.

The separator electrolyte is a first block copolymer electrolyte thatmay be different from the catholyte. The first block of the first blockcopolymer electrolyte may be ionically conductive and may include any ofpolyethers, polyamines, polyimides, polyamides, alkyl carbonates,polynitriles, polysiloxanes, polyphosphazines, polyolefins, polydienes,and combinations thereof. The first block of the first block copolymerelectrolyte may include an ionically-conductive comb polymer, which combpolymer comprises a backbone and pendant groups. The backbone mayinclude one or more of polysiloxanes, polyphosphazines, polyethers,polydienes, polyolefins, polyacrylates, polymethacrylates, andcombinations thereof. The pendants may include any of oligoethers,substituted oligoethers, nitrile groups, sulfones, thiols, polyethers,polyamines, polyimides, polyamides, alkyl carbonates, polynitriles,other polar groups, and combinations thereof.

The second block of the first block copolymer electrolyte may includeany of polystyrene, hydrogenated polystyrene, polymethacrylate,poly(methyl methacrylate), polyvinylpyridine, polyvinylcyclohexane,polyimide, polyamide, polypropylene, polyolefins, poly(t-butyl vinylether), poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(t-butyl vinyl ether), polyethylene, poly(phenylene oxide),poly(2,6-dimethyl-1,4-phenylene oxide) (PXE), poly(phenylene sulfide),poly(phenylene sulfide sulfone), poly(phenylene sulfide ketone),poly(phenylene sulfide amide), polysulfone, fluorocarbons,polyvinylidene fluoride, and copolymers that contain styrene,methacrylate, and/or vinylpyridine.

In one arrangement, the fluorinated catholyte includes anionically-conductive alternating copolymer that has a plurality offluorinated polymer segments and a plurality of non-fluorinated polymersegments. The non-fluorinated polymer segments may include carbonate.The non-fluorinated polymer segments may include PEO or PPO. Thenon-fluorinated polymer segments may include carbonate and PEO. Thenon-fluorinated polymer segments may include amide and PEO. In onearrangement, the PEO has a molecular weight between 200 and 400,000 Da.In one arrangement, the PEO further includes cross-linkable monomerssuch as one or more of oxiranes with pendant epoxide groups, allylgroups, acrylate groups, methacrylate groups, and combinations thereof.

In another arrangement, the fluorinated polymer segments include one ormore of fluoropolyethers, perfluoropolyethers, poly(perfluoroalkylacrylate), poly(perfluoroalkyl methacrylate), polytetrafluoroethylene,polychlorotrifluoroethylene, and polyvinylidene fluoride, andcombinations thereof. The perfluoropolyether may include a segment suchas difluoromethylene oxide, tetrafluoroethylene oxide,hexafluoropropylene oxide, tetrafluoroethyleneoxide-co-difluoromethylene oxide, hexafluoropropyleneoxide-co-difluoromethylene oxide, or a tetrafluoroethyleneoxide-cohexafluoropropylene oxide-co-difluoromethylene oxide segmentsand combinations thereof. The fluorinated polymer segments may havemolecular weights between 200 and 400,000 Da.

In another arrangement, the fluorinated catholyte includes a fluorinatedliquid. The fluorinated liquid may be one or more ofperfluoropolyethers, mono-terminated perfluoropolyethers,diol-terminated perfluoropolyethers, alkylcarbonate-terminatedperfluoropolyethers, alkylcarbamate-terminated perfluoropolyethers,poly(perfluoropolyether)acrylates,poly(perfluoropolyether)methacrylates, polysiloxanes with pendantfluorinated groups, and poly(perfluoropolyether)glycidyl ethers. Thefluorinated liquid may be one or more first polymers such as polymerizedperfluoropolyether-acrylates, perfluoropolyether methacrylates, orperfluoropolyether glycidyl ethers, which is (are) copolymerized withone or more second polymers such as acrylates, methacrylates, orglycidyl ethers. The second polymers may make up less than 10 wt % ofthe fluorinated liquid. The fluorinated liquid may have a molecularweight between 200 Da and 10,000 Da. The fluorinated liquid may becrosslinked. The fluorinated liquid further may also include one or moreadditives such as cyclic organic carbonates, cyclic acetals, organicphosphates, cyclic organic sulfates, and cyclic organic sulfonates. Inone arrangement, the positive electrode further includes a polymermatrix into which the fluorinated liquid is absorbed to form afluorinated polymer gel catholyte.

In another arrangement, the fluorinated catholyte includes a mixture ofperfluoropolyethers, each having either one or two terminal urethanegroups covalently coupled thereto. The perfluoropolyethers may be one ormore of:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether, and x ranges between 0 and 1, subscript “1−x”is the mole fraction of tetrafluoroethyleneoxy groups in theperfluoropolyether, and “1−x” ranges between 0 and 1, n is the averagetotal number of randomly co-distributed difluoromethyleneoxy andtetrafluoroethyleneoxy groups in the perfluoropolyether, and n rangesbetween 1 and 50, X is either hydrogen or fluorine, and R^(F) is aperfluorinated C1-C8 straight or branched alkyl group. R¹ and R² mayeach be chosen independently from short chain straight C1-C4 alkyl, orbranched C1-C4 alkyl, 2-methoxyethyl, 2-(2-methoxy) ethoxyethyl, andcyanoethyl. R¹ and R² may be combined with the N in a C5-C8heterocycloalkyl group such as pyrrolidine, piperidine, morpholine, and4-methylpiperazine. One or both of R¹ and R² may be hydrogen.

In another arrangement, the fluorinated catholyte includes a mixture ofperfluoropolyethers, each having one or two terminal cyclic carbonategroups covalently coupled thereto. The perfluoropolyethers may be one ormore of:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether, and x ranges between 0 and 1, subscript “1−x”is the mole fraction of tetrafluoroethyleneoxy groups in theperfluoropolyether, and “1−x” ranges between 0 and 1, subscript n is theaverage total number of randomly co-distributed difluoromethyleneoxy andtetrafluoroethyleneoxy groups in the perfluoropolyether, and n rangesbetween 1 and 50, X is either hydrogen or fluorine, and R^(F) is aperfluorinated C1-C8 straight or branched alkyl group.

In another arrangement, the fluorinated catholyte includes a pluralityof PEO molecules that have perfluoro functional groups grafted onto atleast a portion of them (the plurality of PEO molecules) to form a graftcopolymer. The perfluoro functional groups may make up between 1 mol %and 20 mol % of the graft copolymer. The perfluoro functional groups mayhave a molecular weight ranging from 200 to 10,000 Da. The perfluorofunctional groups may be any of PFPE, polyvinylene fluoride,polyvinylfluoride, polytetrafluoroethylene, PFA, cyclic perfloro alkanessuch as perfluoro(methylcyclohexane) and perfluoro(methylcyclopentane)and aromatic versions such as pentafluorophenoxide,2,3,5,6-tetrafluorophenol, and combinations thereof. The PEO may have amolecular weight between 10 and 500 KDa.

In one arrangement, the catholyte further includes an ionic liquid.

The alkali metal may be any of lithium, sodium, or magnesium. In onearrangement, the alkali metal includes lithium. The first salt and thesecond salt may be each selected independently from the group consistingof LiTFSI, LiPF₆, LiBF₄, LiClO₄, LiOTf, LiC(Tf)₃, LiBOB, and LiDFOB. Thefirst salt and the second salt may or may not be the same. The negativeelectrode may include a material such as Li, Li—Al, Li—Si, Li—Sn, andLi—Mg. In one arrangement, the negative electrode includes negativeelectrode active material particles, an anolyte, optional electronicallyconducting particles, and optional binder. The negative electrode activematerial may be a material such as silicon, silicon alloys of tin (Sn),nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc(Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth(Bi), antimony (Sb), and chromium (Cr), silicon oxides, siliconcarbides, graphite, and mixtures thereof. The anolyte may include aliquid electrolyte that is immiscible with the separator electrolyte.The anolyte may be a block copolymer electrolyte that is immiscible withthe separator electrolyte.

The positive electrode may include electronically conducting carbon. Thepositive electrode active material may include lithium metal oxides orlithium metal phosphates. The positive electrode active material mayinclude elemental sulfur or sulfur composites with carbon orelectronically-conductive polymer. The positive electrode activematerial may be any of FeS₂, FeOF, FeF₃, FeF₂ and MoO₃, sulfur, lithiumpolysulfides, CuO, Cu₂O, FeO, Fe₂O₃, V₆O₁₃, VO₂, Li_(1+x)V₃O₈ (0≦x≦3),Ag_(x)V₂O₅ (0<x≦2), Cu_(x)V₄O₁₁ (0<x≦3), and VOPO₄, LiCoO₂, LFP, NCM,NCA, or mixtures thereof.

In one embodiment of the invention, the fluorinated catholyte is a blockcopolymer electrolyte that may be different from the block copolymerelectrolyte that is in the separator. The block copolymer catholyteincludes a fluorinated polymer that forms a first block and a secondpolymer that forms a second block, the second polymer having a modulusin excess of 1×105 Pa at 25° C. The first blocks associate to form afirst domain and the second blocks associate to form a second domain,and together, the first domain and the second domain form an orderednanostructure. The block copolymer may be a diblock copolymer or atriblock copolymer.

The first block may include an ionically-conductive alternatingcopolymer that has a plurality of fluorinated polymer segments, and aplurality of non-fluorinated polymer segments. The first block mayinclude a plurality of PEO molecules that have perfluoro functionalgroups grafted onto at least a portion of them (the plurality of PEOmolecules) to form a graft copolymer. The second block of may be any ofpolystyrene, hydrogenated polystyrene, polymethacrylate, poly(methylmethacrylate), polyvinylpyridine, polyvinylcyclohexane, polyimide,polyamide, polypropylene, polyolefins, poly(t-butyl vinyl ether),poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(t-butyl vinyl ether), polyethylene, poly(phenylene oxide),poly(2,6-dimethyl-1,4-phenylene oxide) (PXE), poly(phenylene sulfide),poly(phenylene sulfide sulfone), poly(phenylene sulfide ketone),poly(phenylene sulfide amide), polysulfone, fluorocarbons,polyvinylidene fluoride, and copolymers that contain styrene,methacrylate, and/or vinylpyridine.

In another embodiment of the invention, an electrochemical cell isprovided. The cell has a lithium-containing metal negative electrodefoil, a separator layer that includes a first block copolymerelectrolyte and a first lithium salt, and a positive electrode thatincludes positive electrode active material, binder, optionalelectronically conducting particles, a fluorinated catholyte, and asecond lithium salt. The fluorinated catholyte is immiscible with thefirst block copolymer electrolyte. The separator layer is disposedbetween the negative electrode and the positive electrode andfacilitates ionic communication therebetween.

In one arrangement, the first block of the first block copolymerincludes an ionically-conductive alternating copolymer that has aplurality of fluorinated polymer segments, and a plurality ofnon-fluorinated polymer segments.

In one arrangement, the fluorinated catholyte includes a mixture ofperfluoropolyethers, each having either one or two terminal urethanegroups covalently coupled thereto. In another arrangement, thefluorinated catholyte includes a mixture of perfluoropolyethers, eachhaving one or two terminal cyclic carbonate groups covalently coupledthereto. In another arrangement, the fluorinated catholyte includes aplurality of PEO molecules that have perfluoro functional groups graftedonto at least a portion of them (the plurality of PEO molecules) to forma graft copolymer.

In another arrangement, the fluorinated catholyte includes a secondblock copolymer electrolyte that includes a fluorinated polymer in itsfirst block and a second polymer that forms a second block. The secondpolymer may have a modulus in excess of 1×10⁵ Pa at 25° C. The firstblocks associate to form a first domain and the second blocks associateto form a second domain, and together, the first domain and the seconddomain form an ordered nanostructure. The second block copolymerelectrolyte may be either a diblock copolymer or a triblock copolymer.In one arrangement, the first block (of the second block copolymerelectrolyte) includes an ionically-conductive alternating copolymer thathas a plurality of fluorinated polymer segments and a plurality ofnon-fluorinated polymer segments. In another arrangement, the firstblock (of the second block copolymer electrolyte) includes a pluralityof PEO molecules, onto at least a portion of which perfluoro functionalgroups have been grafted to form a graft copolymer. In one arrangement,the first block copolymer electrolyte and the second block copolymerelectrolyte are the same.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context of electrolytesin an electrochemical cell. The skilled artisan will readily appreciate,however, that the materials and methods disclosed herein will haveapplication in a number of other contexts where optimizingelectrochemical interactions between electrolytes and electrochemicallyactive materials are important. These electrolytes can be useful inelectrochemical devices such as capacitors, electrochemical/capacitivememory, electrochemical (e.g., dye sensitized) solar cells, andelectrochromic devices.

These and other objects and advantages of the present invention willbecome more fully apparent from the following description taken inconjunction with the accompanying drawings.

In this disclosure, the terms “negative electrode,” “NE,” and “anode”are both used to mean “negative electrode.” Likewise, the terms“positive electrode,” “PE,” and “cathode” are both used to mean“positive electrode.”

In this disclosure, the term “dry polymer” is used to mean a polymerwith long chains that has not been plasticized by small molecules.Organic solvents or plasticizers are not added to such dry polymers.

Although not always mentioned explicitly, it should be understood thatelectrolytes, as described herein, include metal salt(s), such aslithium salt(s), to ensure that they are ionically conductive.Non-lithium salts such as other alkali metal salts or salts of aluminum,sodium, or magnesium can also be used. In general, salts that containthe metal ion that shuttles back and forth during electrochemical cellcycling are the ones that are used.

Molecular weights in this disclosure have been determined by theweight-averaged method. Some abbreviations used in this disclosure areshown in Table I below.

TABLE I Abbreviation Meaning MPITFSI 1-methyl-3-propylimidazoliumbis(trifluoromethylsulfonyl)imide PEO poly(ethylene oxide) PFPEperfluoropolyether PFA perfluoro alkane PEG polyethylene glycol PAGEpolyallyl glycidyl ether PPO polypropylene oxide ¹H NMR proton nuclearmagnetic resonance spectroscopy GPC gel permeation chromatography DSCdifferential scanning calorimetry

An electrochemical cell has a negative electrode assembly and a positiveelectrode assembly with an ionically conductive separator in between. Inone embodiment of the invention, the negative electrode assemblycontains at least negative electrode active material and an electrolytethat has been chosen specifically for use with the negative electrodeactive material, referred to herein as the NE (negative electrode)electrolyte.

FIG. 1 illustrates various exemplary arrangements for negative electrodeactive material (black regions) and NE electrolyte or anolyte (greyregions). The negative electrode active material can be arranged asparticles (FIGS. 1a-1d, 1f ) or as a thin film or foil (FIG. 1e ). Thenegative electrode assembly can be formed by combining the negativeelectrode material particles with the NE electrolyte to form a compositelayer (FIGS. 1a-1d ). In some arrangements, other materials (not shown)can be added to the composite layer to enhance, for example, electronicor ionic conduction. In some arrangements, the composite is porous,i.e., contains voids which are shown as white spaces in FIGS. 1b, 1d ;in other arrangements, the composite is pore-free (FIGS. 1a, 1c ). Inyet other arrangements, the NE dry polymer electrolyte of FIGS. 1e and1f may also contain pores (not shown). In a negative electrode assemblythat has a composite layer, the NE dry polymer electrolyte may becontained entirely within the composite layer (FIGS. 1a, 1b ). Inanother arrangement, there can be a thin layer of additional NE drypolymer electrolyte adjacent the composite layer (FIGS. 1c, 1d ). Insome arrangements, a current collector (shown as a white layer definedby dashed lines) is also part of the negative electrode assembly.

In arrangements where the negative electrode active material is a thinfilm or foil, the negative electrode assembly contains at least the thinfilm or foil and a layer of the NE electrolyte adjacent and in ioniccontact with the thin film or foil, as shown in FIG. 1e . In somearrangements, the negative electrode material is not a solid thin film,but instead is arranged as an aggregation of negative electrode activematerial particles in close contact with one another to ensure ionic andelectronic communication among the particles (FIG. 1f ). Such astructure can be made, for example, by pressing and/or by sintering thenegative electrode active material particles. In some arrangements,other materials can be added to the layer of negative electrode materialparticles, for example, to enhance electronic or ionic conductivity. Inone arrangement carbon particles added to enhance electronicconductivity. The negative electrode assembly contains at least the NEelectrolyte layer in ionic communication with the layer of negativeelectrode active material particles. In some arrangements there is alsoa current collector (shown as a white layer defined by dashed lines) inelectronic contact with the negative electrode assembly.

The NE electrolyte is chosen specifically for use with the negativeelectrode active material. In one embodiment of the invention, the NEelectrolyte is a dry polymer (a polymer with long chains that has notbeen plasticized by any small molecules) electrolyte. The NE electrolyteis electrochemically stable against the negative electrode activematerial. That is to say that the NE electrolyte is reductively stableand resistant to continuous chemical and electrochemical reactions whichwould cause the NE electrolyte to be reduced at its interface with thenegative electrode material. The NE electrolyte is resistant toreduction reactions over the range of potentials that theelectrochemical cell experiences under conditions of storage andcycling. Such reduction reactions at the negative electrode wouldincrease cell impedance, thus adversely affecting the performance of thecell and/or the capacity of the cell. In addition, the NE electrolyte ischemically stable against the negative electrode active material.

In one embodiment of the invention, the negative electrode assembly hasa thin film or foil as the negative electrode active material (as shownin FIG. 1e ), and the NE dry polymer electrolyte has a high modulus inorder to prevent dendrite growth from the film during cell cycling. Thethin film or foil may be lithium or lithium alloy, though other metalchemistries are possible, such as sodium or magnesium. Non-lithiummetals and non-lithium metal alloys would be used with correspondingelectrolyte salts that include the same metal as the electrochemicallyactive metal in the negative electrode and with appropriate activematerials in the cathode that can absorb and release the same metalions. Graphite may also be used in combination with lithium salts andlithium-based active materials in the anode for secondary cells. The NEdry polymer electrolyte also has good adhesion to the film or foil toensure easy charge transfer and low interfacial impedance between thelayers. In one arrangement, the NE dry polymer electrolyte is void free.The NE dry polymer electrolyte is electrochemically stable down to thelowest operating potential of the electrode. For example, with Li—Alplanar electrodes, the NE dry polymer electrolyte is stable down to 0.3V vs Li/Li⁺. See Table 2 for other NE active materials and theirassociated potentials. In one arrangement, the NE dry polymerelectrolyte is mechanically rigid enough to prevent continuousreactivity of active material particles that undergo large volumechanges during cell cycling by keeping them in electrical contact withthe matrix of the composite electrode. When negative electrode activematerials that undergo large volume expansion upon absorption of lithiumare used as thin film electrodes, it is useful if the NE dry polymerelectrolyte has high yield strain to prevent electrode fatigue.

In another embodiment of the invention, the negative electrode activematerial is an alloy (examples of which are shown in Table 2) and hasthe form of particles. In order to prevent continuous reactivity, it isuseful if the NE electrolyte is electrochemically stable down to thereduction potentials shown. Additionally it is useful if the NEelectrolyte has high impact toughness in order to maintain mechanicalintegrity and high yield strain in order to accommodate the volumechange of the NE active material particles as they absorb and releaselithium. It is also useful if the NE electrolyte contains voids that canshrink to accommodate expansion. Good compatibility between theelectrolyte and the particle surfaces helps to ensure good adhesion andhomogeneous dispersion. Finally, if a current collector is used, it isuseful if the NE electrolyte can adhere to the current collector.

TABLE 2 Negative Electrode Active Material Characteristics NegativeElectrode Reduction Potential vs. Maximum Volumetric Active MaterialLi/Li+ (volts) Expansion Li—Si 0.4 30%-400% Li—Al 0.3 30%-100% Li—Sn 0.530%-450% graphite 0.2 ~25%

The negative electrode active material can be any of a variety ofmaterials depending on the type of chemistry for which the cell isdesigned. In one embodiment of the invention, the cell is a lithium orlithium ion cell. The negative electrode material can be any materialthat can serve as a host material for (i.e., can absorb and release)lithium ions. Examples of such materials include, but are not limited tographite, lithium metal, and lithium alloys such as Li—Al, Li—Si, Li—Sn,and Li—Mg. In one embodiment of the invention, a lithium alloy thatcontains no more than about 0.5 weight % aluminum is used. Silicon andsilicon alloys are known to be useful as negative electrode materials inlithium cells. Examples include silicon alloys of tin (Sn), nickel (Ni),copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium(In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony(Sb), and chromium (Cr) and mixtures thereof. In some arrangements,graphite, metal oxides, silicon oxides or silicon carbides can also beused as negative electrode materials.

In one embodiment of the invention, a positive electrode assemblycontains at least positive electrode active material and an electrolytethat has been chosen specifically for use with the positive electrodeactive material, referred to herein as a PE (positive electrode)electrolyte. FIG. 2 illustrates various exemplary arrangements for thepositive electrode active material (light grey regions) and the PEelectrolyte or catholyte (dark grey regions). The positive electrodeactive material can be arranged as particles (FIGS. 2a-2d ) or as a thinfilm or foil (FIG. 2e ). The positive electrode assembly can be formedby combining the positive electrode material particles with the PEelectrolyte to form a composite layer (FIGS. 2a-2d ). In somearrangements, other materials (not shown) can be added to the compositelayer to enhance, for example, electronic conduction. In somearrangements, the composite is porous, i.e., contains voids, which areshown as white spaces in FIGS. 2b, 2d ; in other arrangements, it ispore-free (FIGS. 2a, 2c ). In yet other arrangements (not shown), the PEelectrolyte of FIGS. 2e and 2f can also contain pores (not shown). In apositive electrode assembly that has a composite layer, the PEelectrolyte may be contained entirely within the composite layer (FIGS.2a, 2b ). In another arrangement, there can be a thin layer ofadditional PE electrolyte adjacent the composite layer (FIGS. 2c, 2d ).In some arrangements, a current collector (shown as a white layerdefined by dashed lines) is also part of the positive electrodeassembly.

In arrangements where the positive electrode active material is a thinfilm or foil, the positive electrode assembly contains at least the thinfilm or foil and a layer of the PE electrolyte adjacent and in ioniccontact with the thin film or foil as shown in FIG. 2e . In somearrangements, the positive electrode material is not a solid thin film,but instead is arranged as an aggregation of positive electrode activematerial particles close together to ensure ionic and electroniccommunication among the particles (FIG. 2f ). Such a structure can bemade, for example, by pressing and/or by sintering the positiveelectrode active material particles. In some arrangements, othermaterials such as carbon particles can be added to the layer of positiveelectrode material particles, for example, to enhance electronic orionic conductivity. The positive electrode assembly contains at leastthe PE electrolyte layer in ionic communication with the layer ofpositive electrode active material particles. In some arrangements thereis also a current collector (shown as a white layer defined by dashedlines) in electronic contact with the positive electrode.

The PE electrolyte is chosen specifically for use with the positiveelectrode active material. In one embodiment of the invention, the PEelectrolyte is a dry polymer (a polymer with long chains that has notbeen plasticized by any small molecules) electrolyte. The PE electrolyteis chosen to be oxidatively stable against the positive electrode activematerial. That is to say that the PE electrolyte is resistant tocontinuous chemical and electrochemical reactions which would cause thePE electrolyte to be oxidized at its interface with the positiveelectrode material. The PE electrolyte is resistant to oxidationreactions over the range of potentials that the electrochemical cellexperiences under conditions of storage and cycling. Such oxidationreactions at the positive electrode would increase cell impedance, thusadversely affecting the performance of the cell and/or the capacity ofthe cell. In addition, the PE electrolyte is chemically stable againstthe positive electrode active material.

The positive electrode active material can be any of a variety ofmaterials depending on the type of chemistry for which the cell isdesigned. In one embodiment of the invention, the cell is a lithium orlithium ion cell. The positive electrode active material can be anymaterial that can serve as a host material for lithium ions. Examples ofsuch materials include, but are not limited to materials described bythe general formula Li_(x)A_(1−y)M_(y)O₂, wherein A comprises at leastone transition metal selected from the group consisting of Mn, Co, andNi; M comprises at least one element selected from the group consistingof B, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, In, Nb, Mo, W,Y, and Rh; x is described by 0.05≦x≦1.1; and y is described by 0≦y≦0.5.In one arrangement, the positive electrode material isLiNi_(0.5)Mn_(0.5)O₂.

In one arrangement, the positive electrode active material is describedby the general formula: Li_(x)Mn_(2−y)M_(y)O₂, where M is chosen fromMn, Ni, Co, and/or Cr; x is described by 0.05≦x≦1.1; and y is describedby 0≦y≦2. In another arrangement, the positive electrode active materialis described by the general formula: Li_(x)M_(y)Mn_(4−y)O₈, where M ischosen from Fe and/or Co; x is described by 0.05≦x≦2; and y is describedby 0≦y≦4. In another arrangement, the positive electrode active materialis given by the formula Li_(x)(Fe_(y)M_(1−y))PO₄, where M is chosen fromtransition metals such as Mn, Co and/or Ni; x is described by 0.9≦x≦1.1;and y is described by 0≦y≦1. In yet another arrangement, the positiveelectrode active material is given by the general formula:Li(Ni_(0.5−x)Co_(0.5−x)M_(2x))O₂, where M is chosen from Al, Mg, Mn,and/or Ti; and x is described by 0≦x≦0.2. In some arrangements, thepositive electrode material includes LiNiVO₂.

Examples of appropriate positive electrode active materials also includecompounds such as, FeS₂, FeOF, FeF₃, FeF₂, MoO₃, sulfur, lithiumpolysulfides, CuO, Cu₂O, FeO, Fe₂O₃, V₆O₁₃, VO₂, Li_(1+x)V₃O₈ (0≦x≦3),Ag_(x)V₂O₅ (0<x≦2), Cu_(x)V₄O₁₁ (0<x≦3), VOPO₄, LiCoO₂, lithium ironphosphate (LFP), nickel-cobalt manganese (NCM), nickel cobalt aluminum(NCA), or mixtures thereof.

Most electrolytes exhibit electrochemical stability over a limitedwindow of about 4 Volts. Thus a single electrolyte cannot by itselfsupport an electrochemical couple that has a voltage between electrodeshigher than 4 Volts. Yet such high voltage electrochemical cells can bemade to be stable and robust using the structures and materialsdescribed herein. Two different electrolytes—a NE electrolyte that isreductively stable at the anode (but may or may not be oxidativelystable at the cathode) and a PE electrolyte that is oxidatively stableat the cathode (but may or may not be reductively stable at the anode)can now be used in the same electrochemical cell. In one embodiment ofthe invention, the NE electrolyte is optimized for reductive stabilityand the PE electrolyte is optimized for oxidative stability. By allowingdifferent electrolytes to be used at the negative electrode and at thepositive electrode, each electrode can be designed for optimumperformance without compromise. Such an arrangement is especially usefuland for high voltage applications.

There have been efforts in recent years to develop high voltage (i.e.,greater than ˜4.2V) electrochemical cells by using “high voltage cathodematerials” such as those listed in Table 3.

TABLE 3 Positive Electrode Active Material Characteristics TypicalCharging cut Positive Electrode Discharge Potential vs. off PotentialLi/Li+ Active Material Li/Li+ (volts) (volts) LiFePO₄ 3.4 3.8 LiCoO₂ 3.64.2 LiMnPO₄ 4.1 4.3 LiAl_(0.05)Co_(0.15)Ni_(0.8)O₂ 3.6 4.3 LiCoPO₄ 4.85.0 LiNiPO₄ 5.1 5.4 Li_(1.07)Mn_(1.93)O₄ 3.9 4.3Unfortunately, electrolytes that are stable to oxidation at the highpotentials at the cathode/electrolyte interface are not generally stableto reduction at the lower potentials at the anode/electrolyte interfacefor standard anode materials. Now an electrochemical cell that usesdifferent, specifically chosen electrolytes, some of which may be drypolymer electrolytes, at the cathode and at the anode sides of the cell,as described herein, can overcome this problem and make it possible todesign and build high voltage cells.

Lithium metal and alloy negative electrode active materials areparticularly prone to ongoing reduction reactions with many conventionallithium-ion electrolytes, as these negative electrode active materialstend not to form stable passivation layers. Although some electrolytesmay be able to form stable interfaces with such anode materials, suchelectrolytes may not work well in the rest of the cell or in thepositive electrode assembly due to limitations in conductivity and/oroxidative stability. Electrochemical cells that can use differentelectrolytes specifically chosen for their compatibility with eachelectrode, as described in the embodiments herein, can overcome theselimitations.

The embodiments of the invention, as described above, can result in anelectrochemical cell with very good performance. In one embodiment ofthe invention, such a cell has a Li cycling efficiency greater than99.7%, over 500 cycles. In another embodiment of the invention, such acell has a Li cycling efficiency of greater than 99.9%, over 500 cycles.In another embodiment of the invention, there is very little impedanceincrease at the negative electrode, the positive electrode, or at bothelectrodes as the cell is cycled. In one arrangement, the impedancevalue at 500 cycles increases by no more than 40% from the impedancevalue at 10 cycles. In another arrangement, the impedance value at 500cycles increases by no more than 20% from the impedance value at 10cycles. In yet another arrangement, the impedance value at 500 cyclesincreases by no more than 10% from the impedance value at 10 cycles. Inone embodiment of the invention, the capacity of the electrolyte cell at500 cycles decreases by no more than 40% from the capacity at 10 cycles.In another embodiment of the invention, the capacity of the electrolytecell at 500 cycles decreases by no more than 20% from the capacity at 10cycles. In yet another embodiment of the invention, the capacity of theelectrolyte cell at 500 cycles decreases by no more than 10% from thecapacity at 10 cycles. In yet another embodiment of the invention, thecapacity of the electrolyte cell at 500 cycles decreases by no more than5% from the capacity at 10 cycles.

When negative and positive electrode assemblies are each optimizedindependently, not only is it possible to optimize electrochemicalstability, but it also presents the opportunity to overcome other keylimitations that may be specific to individual electrode activematerials.

For example, some negative electrode active materials undergo a largevolume increase, as much as 300% or more, upon lithiation. Some examplesare shown above in Table 2. For composite negative electrode assembliesthat contain voids such as the electrode assemblies in FIGS. 1b, 1d , itis possible to accommodate volumetric expansion and contraction of thenegative electrode active material upon cycling. It is useful if the NEelectrolyte is a dry polymer electrolyte that has a yield strain greaterthan or equal to the maximum volume expansion of the negative electrodematerial. In this way, the NE electrolyte is elastic enough to move intothe void space as the negative electrode active material expands. It isalso useful if the total void space is at least as large as the maximumtotal volume expansion of the negative electrode active material. Inother arrangements, the negative electrode material particles are shapedinto a porous layer adjacent the NE electrolyte layer to form thenegative electrode assembly as shown in FIG. 1f . The pores in the layercan accommodate expansion of the negative electrode active material.Further details about porous electrodes can be found in U.S. Pat. No.9,054,372, issued Jun. 9, 2015, which is included by reference herein.

In general, cathode active materials expand and contract much lessduring cell cycling than do anode active materials. Thus there aredifferent mechanical considerations when choosing an electrolyte for acathode rather than for an anode, and it may be desirable to choosedifferent electrolytes for these two regions of an electrochemical cell.For example, if the positive electrode active material expands andcontracts much less than the negative electrode active material, it maybe optimal to employ an electrolyte that is less elastic for the cathoderegion of the cell or to create an electrode assembly for the cathodethat does not include voids, thereby optimizing other key parameters inthe cathode assembly such as mechanical robustness or energy density.One key factor in determining a good PE dry polymer electrolyte iswhether the electrolyte can bind and keep the positive active materialparticles and any electronically conductive additives (e.g., carbonparticles) intermixed and randomly dispersed through the manufacturing(e.g., casting, calendering) process despite significant difference inthe densities of the particles.

For positive electrode active materials that contain transition metals,dissolution of these metals into a standard liquid electrolyte uponcycling can be a serious problem, especially in high voltage cells andat high temperatures. The dissolution can cause accelerated celldegradation or premature failure. Examples of possible failuremechanisms include:

-   -   a) the composition of the positive electrode active materials        changes as the metals dissolve, adversely impacting the ability        of the active material to absorb and release lithium,    -   b) the dissolved metals can diffuse to the negative electrode        and degrade the capacity of th negative electrode active        material,    -   c) the dissolved metals can diffuse to the negative electrode        and degrade any passivation layer on the negative electrode        active material, resulting in continual electrolyte reaction        with the negative electrode active material, and    -   d) the dissolved metals can create internal shorts or other        defects within the cell.

For example, in the case of Mn₂O₄ positive electrode active material, itis useful if the electrolyte does not dissolve the electrochemicallyactive manganese. In the case of a sulfur cathode, it is useful if theelectrolyte does not dissolve the electrochemically active sulfur orpolysulfide. In one arrangement, less than 10% of the electrochemicallyactive ion dissolves from the positive electrode active material after500 cycles in the temperature range 45-80° C. In another arrangement,less than 5% of the electrochemically active ion dissolves from thepositive electrode active material after 500 cycles in the temperaturerange 45-80° C. In yet another arrangement, less than 1% of theelectrochemically active ion dissolves from the positive electrodeactive material after 500 cycles in the temperature range 45-80° C. Thisallows for selection of a separate non-dissolving electrolyte on thecathode side and can prevent diffusion of metal to the anode. A positiveelectrode assembly can be optimized to prevent dissolution, for example,employing a ceramic or solid polymer electrolyte as the PE electrolyte.Although dissolution of electrochemically active ions may not be anissue for the negative electrode assembly, other considerations may beimportant, such as high ionic conductivity or reductively stability, andit may be possible that a different electrolyte would be preferred.

In one embodiment of the invention, the NE electrolyte and/or the PEelectrolyte is a solid electrolyte. In one arrangement, the NEelectrolyte and/or the PE electrolyte is a ceramic electrolyte. Inanother arrangement, the NE electrolyte and/or the PE electrolyte is adry polymer electrolyte. In yet another arrangement, the NE electrolyteand/or the PE electrolyte is a dry block copolymer electrolyte.

In one embodiment of the invention, the NE electrolyte and/or the PEelectrolyte is a liquid electrolyte or a gel containing a liquidelectrolyte. When a liquid electrolyte is used, it is most useful if theliquid electrolyte is immiscible with electrolytes in adjacent regionsof the cell or if a selectively permeable membrane is positioned toprevent mixing of the liquid electrolyte with adjacent electrolytes.Such a membrane allows electrochemical cations to move through, but notthe liquid itself. In the absence of containment by such a membrane,miscible liquids can diffuse easily throughout the cell. If suchdiffusion were to occur, the benefits provided by using differentelectrolytes in different regions of the cell may be diminished ornegated. In the worst case, active materials in the electrodes could beoxidized or reduced, seriously compromising the performance and/or thelife of the cell.

In one embodiment of the invention, a separator electrolyte is usedbetween the negative electrode assembly and the positive electrodeassembly. In one embodiment of the invention, the separator electrolytecan be the same as either the NE electrolyte or as the PE electrolyte.In another embodiment, the separator electrolyte is different from boththe NE electrolyte and the PE electrolyte. The separator electrolyte canbe any of liquid electrolytes, solid electrolytes, ceramic electrolytes,polymer electrolytes, dry polymer electrolytes, and block copolymerelectrolytes, independent of the NE electrolyte and the PE electrolyte.In some arrangements, the electrolytes are chosen so that no two liquidelectrolytes are adjacent one another. When a liquid electrolyte isused, it is most useful if the liquid electrolyte is immiscible withelectrolytes in adjacent regions of the cell or if a selective membraneis positioned at each interface to prevent mixing of the liquidelectrolyte with adjacent electrolytes. Such a membrane allowselectrochemical cations to move through but not the liquid itself. Inthe absence of containment, miscible liquids can diffuse easilythroughout the cell. If such diffusion were to occur, the benefitsprovided by using different electrolytes in different regions of thecell may be diminished or negated. In the worst case, such diffusioncould cause reduction at the negative electrode assembly and/oroxidation at the positive electrode assembly, causing premature failureof the cell.

In general, it is useful if the separator electrolyte has enoughmechanical integrity to ensure that the negative electrode assembly andthe positive electrode assembly do not come into physical contact withone another. In some arrangements, when a liquid, gel, or soft polymeris used as the separator electrolyte, a separator membrane is used withit.

It is useful if any two electrolytes meeting at an interface areimmiscible in each other and chemically compatible with each other. Itis also useful if there is little or no impedance or concentrationoverpotential across the interface.

In one arrangement, all electrolytes are stable over the range ofstorage and operating temperatures and the range of operating potentialsfor the electrochemical cell. Using the embodiments described here, thiscondition can be met for electrode couples that are otherwise unstablewith conventional electrolytes or in conventional single-electrolytearchitectures.

FIG. 3 is a schematic cross section that shows an electrochemical cellin an exemplary embodiment of the invention. The cell 300 has a negativeelectrode assembly 310, a positive electrode assembly 320, with anintervening separator 330. The exemplary negative electrode assembly 310is the same as the one shown in FIG. 1a . The negative electrodeassembly 310 is an aggregation of negative electrode active materialparticles 314 dispersed within a NE dry polymer electrolyte 318. Therecan also be electronically-conducting particles such as carbon particles(not shown) in the negative electrode assembly 310. The exemplarypositive electrode assembly 320 is the same as the one shown in FIG. 2a. The positive electrode assembly 320 is an aggregation of positiveelectrode active material particles 324 dispersed within a PE drypolymer electrolyte 328. There can also be electronically-conductingparticles such as carbon particles (not shown) in the positive electrodeassembly 320. In other exemplary embodiments, other electrode assemblyconfigurations, such as those shown in FIGS. 1 and 2, can be substitutedin the electrochemical cell shown in FIG. 3.

The NE electrolyte 318 and the PE electrolyte 328 are each optimized fortheir respective electrodes as has been discussed above. In onearrangement, the NE electrolyte 318 and the PE electrolyte 328 aredifferent. In another arrangement, the NE electrolyte 318 and the PEelectrolyte 328 are the same. The separator 330 contains a separatorelectrolyte 338, which is also optimized for its role in the cell 300.In one arrangement, the separator electrolyte 338 is immiscible withboth the NE electrolyte 318 and the PE electrolyte 328. In anotherarrangement, the separator electrolyte 338 is miscible with either orboth of the NE electrolyte 318 and the PE electrolyte 328, andselectively permeable membranes (not shown) are positioned at interfacesbetween the miscible electrolytes. In one arrangement, the separatorelectrolyte 338 is the same as either the NE electrolyte 318 or the PEelectrolyte 328. In another arrangement, the separator electrolyte 338is different from both the NE electrolyte 318 and the PE electrolyte328.

In one arrangement, the NE electrolyte 318, the PE electrolyte 328, andthe separator electrolyte 338 are all solid electrolytes. In somearrangements, solid electrolytes can be made of ceramic materials orpolymer materials. In one arrangement, solid electrolytes can be made ofdry polymer materials. In one arrangement, the solid electrolytes areblock copolymer electrolytes. In some arrangements, one or more of theNE electrolyte 318, the PE electrolyte 328, and the separatorelectrolyte 338 is a liquid. When a liquid electrolyte is used, caremust be taken to ensure that the liquid cannot diffuse out of its ownfunctional region (i.e., negative electrode assembly, positive electrodeassembly, or separator) into other functional regions of the cell. Insome arrangements, a selectively permeable membrane is used at anyinterface where at least one electrolyte is liquid. In otherarrangements, the liquid electrolytes that are used are immiscible withany adjacent electrolyte.

FIG. 4 is a schematic cross section that shows an electrochemical cellin another exemplary embodiment of the invention. The cell 400 has anegative electrode 410, a positive electrode assembly 420, with anintervening separator 430. The exemplary negative electrode 410 is alithium metal or lithium metal alloy foil. The exemplary positiveelectrode assembly 420 is an aggregation of positive electrode activematerial particles 424 held together by a binder (not shown) such as oneor more of PVDF, P(HFP-VDF), P(CTFE-VDF), carboxymethylcellulose, andstyrene-butadiene rubber, and surrounded by a fluorinated liquidelectrolyte 428. There can also be electronically-conducting particlessuch as carbon particles (not shown) in the positive electrode assembly420. The electronically-conducting particles may be acetylene black,vapor-grown carbon fiber, or graphite powder, and are present insufficient quantity to allow electronic conduction throughout thecathode.

The separator 430 contains a separator electrolyte 438, which isimmiscible with the PE electrolyte 428. In another arrangement, theseparator electrolyte 438 is miscible with the PE electrolyte 428, and aselectively permeable membrane (not shown) is positioned at theinterface between the miscible electrolytes. In one arrangement, theseparator electrolyte 438 is a block copolymer electrolyte as discussedabove. In one arrangement, the separator electrolyte 438 is a diblock ortriblock copolymer wherein one block is poly(ethylene oxide) to provideionic conduction and the other block is poly(styrene) or otherphysically robust polymer providing structural support. In somearrangements, the separator electrolyte 438 is made of ceramic materialsor polymer materials. In one arrangement, the separator electrolyte 438can be made of dry polymer materials. In one arrangement, the solidelectrolytes are block copolymer electrolytes. In some arrangements, theseparator electrolyte 438 is a liquid. When a liquid electrolyte isused, care must be taken to ensure that the liquid cannot diffuse out ofthe separator region into other functional regions of the cell. In somearrangements, a selectively permeable membrane is used at any interfacewhere at least one electrolyte is liquid. In other arrangements, theliquid electrolytes that are used are immiscible with any adjacentelectrolyte.

The metal salt in the separator is typically a lithium salt with aweakly coordinating anion, such as LiTFSI, LiPF₆, LiBF4, LiClO₄, LiOTf,LiC(Tf)₃, LiBOB, LiDFOB, LiB(CN)₄ among others.

The active material in the cathode is selected from the lithium metaloxides or lithium metal phosphates typically used for lithium batteries.It may be possible to use conversion electrodes such as elementalsulfur, or sulfur composites with carbon or ionically-conductivepolymer.

The metal salt in the cathode is typically identical to one or more ofthe salts present in the block copolymer separator. A fluorinatedcounter-ion is more likely to be soluble at useful levels in thefluorinated liquid such as many of the salts listed above.

Electrolytes that can be Used in the Embodiments of the Invention

Ceramic Electrolytes

Examples of ceramic electrolytes that can be used in the embodiments ofthe invention include lithium silicate, lithium borate, lithiumaluminate, lithium phosphate, lithium phosphorus oxynitride, lithiumsilicosulfide, lithium borosulfide, lithium aluminosulfide, and lithiumphosphosulfide. Other examples include lithium lanthanum titanium oxide,lithium lanthanum zirconium oxide, LiPON, LiSICON, Li₁₀SnP₂S₁₂,Li₁₁Si₂PS₁₂, Li₁₀GeP₂S₁₂, Li₂S—SiS₂—Li₃PO₄, Li₁₄Zn(GeO₄)₄, Li₂S—P₂S₅,La_(0.5)Li_(0.5)TiO₃, combinations thereof, and others known in thefield.

Polymer Electrolytes

There are a variety of polymer electrolytes that are appropriate for usein the inventive structures described herein. In one embodiment of theinvention, an electrolyte contains one or more of the followingoptionally cross-linked polymers: polyethylene oxide, polysulfone,polyacrylonitrile, siloxane, polyether, polyamine, linear copolymerscontaining ethers or amines, ethylene carbonate, Nafion®, andpolysiloxane grafted with small molecules or oligomers that includepolyethers and/or alkylcarbonates.

In one embodiment of the invention, the solid polymer electrolyte, whencombined with an appropriate salt, is chemically and thermally stableand has an ionic conductivity of at least 10⁻⁵ Scm⁻¹ at a desiredoperating temperature. In one arrangement, the polymer electrolyte hasan ionic conductivity of at least 10⁻³ Scm⁻¹ at operating temperature.Examples of useful operating temperatures include room temperature (25°C.), and 80° C. Examples of appropriate salts for any electrolytedisclosed herein include, but are not limited to metal salts selectedfrom the group consisting of chlorides, bromides, sulfates, nitrates,sulfides, hydrides, nitrides, phosphides, sulfonamides, triflates,thiocynates, perchlorates, borates, or selenides of lithium, sodium,potassium, silver, barium, lead, calcium, ruthenium, tantalum, rhodium,iridium, cobalt, nickel, molybdenum, tungsten or vanadium. Alkali metalsalts such as lithium salts, sodium salts, potassium salts, and cesiumsalts can be used. Examples of specific lithium salts include LiSCN,LiN(CN)₂, LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃, Li(CF₃SO₂)₂N,Li(CF₃SO₂)₃C, LiN(SO₂C₂F₅)₂, LiN(SO₂CF₃)₂, LiN(SO₂CF₂CF₃)₂, lithiumalkyl fluorophosphates, lithium oxalatoborate, as well as other lithiumbis(chelato)borates having five to seven membered rings, LiPF₃(C₂F₅)₃,LiPF₃(CF₃)₃, LiB(C₂O₄)₂, LiOTf, LiC(Tf)3, LiDFOB, LiTCB and mixturesthereof. In other embodiments of the invention, for otherelectrochemistries, electrolytes are made by combining the polymers withvarious kinds of salts. Examples include, but are not limited toAgSO₃CF₃, NaSCN, NaSO₃CF₃, KTFSI, NaTFSI, Ba(TFSI)₂, Pb(TFSI)₂, andCa(TFSI)₂. Concentration of metal salts in the electrolytes disclosedherein range from 5 to 50 wt %, 5 to 30 wt %, 10 to 20 wt %, or anyrange subsumed therein. As described in detail above, a block copolymerelectrolyte can be used in the embodiments of the invention.

Block Copolymer Electrolytes

FIG. 5A is a simplified illustration of an exemplary diblock polymermolecule 500 that has a first polymer block 510 and a second polymerblock 520 covalently bonded together. In one arrangement both the firstpolymer block 510 and the second polymer block 520 are linear polymerblocks. In another arrangement, either one or both polymer blocks 510,520 has a comb structure. In one arrangement, neither polymer block iscross-linked. In another arrangement, one polymer block is cross-linked.In yet another arrangement, both polymer blocks are cross-linked.

Multiple diblock polymer molecules 500 can arrange themselves to form afirst domain 515 of a first phase made of the first polymer blocks 510and a second domain 525 of a second phase made of the second polymerblocks 520, as shown in FIG. 5B. Diblock polymer molecules 500 canarrange themselves to form multiple repeat domains, thereby forming acontinuous nanostructured block copolymer material 540, as shown in FIG.5C. The sizes or widths of the domains can be adjusted by adjusting themolecular weights of each of the polymer blocks.

In one arrangement the first polymer domain 515 is ionically conductive,and the second polymer domain 525 provides mechanical strength to thenanostructured block copolymer.

FIG. 6A is a simplified illustration of an exemplary triblock polymermolecule 600 that has a first polymer block 610 a, a second polymerblock 620, and a third polymer block 610 b that is the same as the firstpolymer block 610 a, all covalently bonded together. In one arrangementthe first polymer block 610 a, the second polymer block 620, and thethird copolymer block 610 b are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 610 a, 620, 610 b have acomb structure. In one arrangement, no polymer block is cross-linked. Inanother arrangement, one polymer block is cross-linked. In yet anotherarrangement, two polymer blocks are cross-linked. In yet anotherarrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 600 can arrange themselves to form afirst domain 615 of a first phase made of the first polymer blocks 610a, a second domain 625 of a second phase made of the second polymerblocks 620, and a third domain 615 b of a first phase made of the thirdpolymer blocks 610 b as shown in FIG. 6B. Triblock polymer molecules 600can arrange themselves to form multiple repeat domains 625, 615(containing both 615 a and 615 b), thereby forming a continuousnanostructured block copolymer 630, as shown in FIG. 6C. The sizes ofthe domains can be adjusted by adjusting the molecular weights of eachof the polymer blocks.

In one arrangement the first and third polymer domains 615 a, 615 b areionically conductive, and the second polymer domain 625 providesmechanical strength to the nanostructured block copolymer. In anotherarrangement, the second polymer domain 625 is ionically conductive, andthe first and third polymer domains 615 provide a structural framework.

FIG. 7A is a simplified illustration of another exemplary triblockpolymer molecule 700 that has a first polymer block 710, a secondpolymer block 720, and a third polymer block 730, different from eitherof the other two polymer blocks, all covalently bonded together. In onearrangement the first polymer block 710, the second polymer block 720,and the third copolymer block 730 are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 710, 720, 730 have a combstructure. In one arrangement, no polymer block is cross-linked. Inanother arrangement, one polymer block is cross-linked. In yet anotherarrangement, two polymer blocks are cross-linked. In yet anotherarrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 700 can arrange themselves to form afirst domain 715 of a first phase made of the first polymer blocks 710a, a second domain 725 of a second phase made of the second polymerblocks 720, and a third domain 735 of a third phase made of the thirdpolymer blocks 730 as shown in FIG. 7B. Triblock polymer molecules 700can arrange themselves to form multiple repeat domains, thereby forminga continuous nanostructured block copolymer 740, as shown in FIG. 7C.The sizes of the domains can be adjusted by adjusting the molecularweights of each of the polymer blocks.

In one arrangement the first polymer domains 715 are ionicallyconductive, and the second polymer domains 725 provide mechanicalstrength to the nanostructured block copolymer. The third polymerdomains 735 provides an additional functionality that may improvemechanical strength, ionic conductivity, chemical or electrochemicalstability, may make the material easier to process, or may provide someother desirable property to the block copolymer. In other arrangements,the individual domains can exchange roles.

Choosing appropriate polymers for the block copolymers described aboveis important in order to achieve desired electrolyte properties. In oneembodiment, the conductive polymer exhibits ionic conductivity of atleast 10⁻⁵ Scm⁻¹ at electrochemical cell operating temperatures whencombined with an appropriate salt(s), such as lithium salt(s); ischemically stable against such salt(s); and is thermally stable atelectrochemical cell operating temperatures. In one embodiment, thestructural material has a modulus in excess of 1×10⁵ Pa atelectrochemical cell operating temperatures. In one embodiment, thethird polymer is rubbery; and has a glass transition temperature lowerthan operating and processing temperatures. It is useful if allmaterials are mutually immiscible.

In one embodiment of the invention, the conductive phase can be made ofa linear polymer. Conductive linear polymers that can be used in theconductive phase include, but are not limited to, polyethers,polyamines, polyimides, polyamides, alkyl carbonates, polynitriles, andcombinations thereof. The conductive linear polymers can also be used incombination with polysiloxanes, polyphosphazines, polyolefins, and/orpolydienes to form the conductive phase.

In another exemplary embodiment, the conductive phase is made of combpolymers that have a backbone and pendant groups. Backbones that can beused in these polymers include, but are not limited to, polysiloxanes,polyphosphazines, polyethers, polydienes, polyolefins, polyacrylates,polymethacrylates, and combinations thereof. Pendants that can be usedinclude, but are not limited to, oligoethers, substituted oligoethers,nitrile groups, sulfones, thiols, polyethers, polyamines, polyimides,polyamides, alkyl carbonates, polynitriles, other polar groups, singleion conducting groups, and combinations thereof.

Further details about polymers that can be used in the conductive phasecan be found in International Patent Application Publication Number WO2009/146340, published Dec. 23, 2009, U.S. Pat. No. 8,691,928, issuedApr. 8, 2014, International Patent Application Publication Number WO2010083325, published Jul. 22, 2010, International Patent ApplicationPublication Number WO 2010/083330, published Jul. 22, 2010, U.S. Pat.No. 9,048,507, issued Jun. 2, 2015, and U.S. Pat. No. 8,598,273, issuedDec. 3, 2013, all of which are included by reference herein.

There are no particular restrictions on the electrolyte salt that can beused in the block copolymer electrolytes. Any electrolyte salt thatincludes the ion identified as the most desirable charge carrier for theapplication can be used. It is especially useful to use electrolytesalts that have a large dissociation constant within the polymerelectrolyte.

Suitable examples include alkali metal salts, such as Li salts. Examplesof useful Li salts include, but are not limited to, LiPF₆, LiN(CF₃SO₂)₂,Li(CF₃SO₂)₃C, LiN(SO₂CF₂CF₃)₂, LiB(C₂O₄)₂, B₁₂F_(x)H_(12−x), B₁₂F₁₂, andmixtures thereof. Non-lithium salts such as salts of aluminum, sodium,and magnesium are examples of other salts that can be used.

In one embodiment of the invention, single ion conductors can be usedwith electrolyte salts or instead of electrolyte salts. Examples ofsingle ion conductors include, but are not limited to sulfonamide salts,boron based salts, and sulfates groups.

In one embodiment of the invention, the structural phase can be made ofpolymers such as polystyrene, hydrogenated polystyrene,polymethacrylate, poly(methyl methacrylate), polyvinylpyridine,polyvinylcyclohexane, polyimide, polyamide, polypropylene, polyolefins,poly(t-butyl vinyl ether), poly(cyclohexyl methacrylate),poly(cyclohexyl vinyl ether), poly(t-butyl vinyl ether), polyethylene,poly(phenylene oxide), poly(2,6-dimethyl-1,4-phenylene oxide) (PXE),poly(phenylene sulfide), poly(phenylene sulfide sulfone), poly(phenylenesulfide ketone), poly(phenylene sulfide amide), polysulfone,fluorocarbons, such as polyvinylidene fluoride, or copolymers thatcontain styrene, methacrylate, or vinylpyridine. It is especially usefulif the structural phase is rigid and is in a glassy or crystallinestate.

Additional species can be added to nanostructured block copolymerelectrolytes to enhance the ionic conductivity, to enhance themechanical properties, or to enhance any other properties that may bedesirable.

The ionic conductivity of nanostructured block copolymer electrolytematerials can be improved by including one or more additives in theionically conductive phase. An additive can improve ionic conductivityby lowering the degree of crystallinity, lowering the meltingtemperature, lowering the glass transition temperature, increasing chainmobility, or any combination of these. A high dielectric additive canaid dissociation of the salt, increasing the number of Li+ ionsavailable for ion transport, and reducing the bulky Li+[salt] complexes.Additives that weaken the interaction between Li+ and PEO chains/anions,thereby making it easier for Li+ ions to diffuse, may be included in theconductive phase. The additives that enhance ionic conductivity can bebroadly classified in the following categories: low molecular weightconductive polymers, ceramic particles, room temp ionic liquids (RTILs),high dielectric organic plasticizers, and Lewis acids.

Other additives can be used in the polymer electrolytes describedherein. For example, additives that help with overcharge protection,provide stable SEI (solid electrolyte interface) layers, and/or improveelectrochemical stability can be used. Such additives are well known topeople with ordinary skill in the art. Additives that make the polymerseasier to process, such as plasticizers, can also be used.

In one embodiment of the invention, neither small molecules norplasticizers are added to the block copolymer electrolyte and the blockcopolymer electrolyte is a dry polymer.

Further details about block copolymer electrolytes are described in U.S.Pat. No. 8,563,168, issued Oct. 22, 2013, U.S. Pat. No. 8,268,197,issued Sep. 18, 2012, and U.S. Pat. No. 8,889,301, issued Nov. 18, 2014,all of which are included by reference herein.

Fluorinated Polymer Electrolytes

Fluorinated polymer electrolytes can be used in cells that have multipleelectrolyte layers. Fluorinated electrolytes are typically immisciblewith non-fluorinated electrolytes, so they can be confined to one regionor layer of a cell easily. In various embodiments fluorinatedelectrolytes may include polymers, dry polymers, liquids, and gels, allof which can be used cells that have multiple electrolyte layers.

In one embodiment of the invention, an electrolyte made from anionically-conductive alternating copolymer that includes bothfluorinated polymer segments and non-fluorinated polymer segments isused either alone or as one block of a block copolymer electrolyte, forexample with a second rigid mechanical block. The non-fluorinatedsegments may be carbonate or PEO, or both. If the molecular weight ofthe copolymer is low (for example, less than 5000 Da), the copolymer maybe a liquid at operating temperatures. At high molecular weights, thecopolymer may be a solid. In one embodiment, a liquid plasticizer isadded to the copolymer to make a gel electrolyte. Exemplary liquidadditives include fluorinated materials and room-temperature ionicliquids.

Formation of PFPE-PEO Alternating Copolymers

In one embodiment of the invention, an alternating copolymer based onPFPE and PEO can be obtained by reacting a PFPE-diol (nucleophile) withan electrophilic PEG molecule as shown in Scheme 1 below. This reactionuses a base to activate the alcohols in PFPE. The molecular weight ofthe resulting copolymer can be tuned by controlling the stoichiometrybetween the PFPE nucleophile and PEO-based electrophile. The relativeamounts of PFPE and PEG in the final copolymer can be controlled byvarying the molecular weight of the two components. The PEO may have amolecular weight between 200 and 400,000 Da or any range subsumedtherein. The fluorinated polymer segments may have molecular weightsbetween 200 and 400,000 Da or any range subsumed therein. PFPE-PEOalternating copolymers may be solid, gels, or liquids depending on theirmolecular weights.

Scheme 1 below can be used to synthesize other variations of PEG or PEOsuch as polypropylene oxide (PPO) or polyallyl glycidyl ether (PAGE).Values for r can range from 1 to 10,000; for s from 1 to 10,000; and fort from 1 to 10,000. Also, PEO with small amounts of cross-linkablemonomers can be utilized to achieve a cross-linked electrolyte. Examplesof such cross-linkable monomers (such as X) include, but are not limitedto, oxiranes with pendant epoxide groups, allyl groups, acrylate groups,methacrylate groups, and combinations thereof.

In one embodiment of the invention, an alternating copolymer based onPFPE and PEO can be obtained by reacting a PFPE-methyl ester with PEGdiamine molecule as shown in Scheme 2 below. This reaction uses aminefunction groups on PEG to react with methyl esters on PFPE to form amidelinkages. The molecular weight of the resulting copolymer can be tunedby controlling the stoichiometry between the PFPE methyl ester andPEO-based diamine. The relative amounts of PFPE and PEG in the finalcopolymer can be controlled by varying the molecular weights of the twocomponents. The PEO may have a molecular weight between 200 and 400,000Da or any range subsumed therein. The fluorinated polymer segments mayhave molecular weights between 200 and 400,000 Da or any range subsumedtherein. PFPE-PEO alternating copolymers may be solid, gels, or liquidsdepending on their molecular weights.

Scheme 2 below can be used to synthesize variations of PEG or PEO suchas polypropylene oxide (PPO) or polyallyl glycidyl ether (PAGE) withdiamine functional groups. The PEG or PEO diamine can be reacted withester-functionalized PFPE to form amide linkages between the PEG or PEOand the PFPE. Values for r can range from 1 to 10,000; for s from 1 to10,000; and for t from 1 to 10,000. Also, PEO or PEG with small amountsof cross-linkable monomers (such as X) can be utilized to achieve across-linked electrolyte. Examples of such cross-linkable monomersinclude, but are not limited to, oxiranes with pendant epoxide groups,allyl groups, acrylate groups, methacrylate groups, and combinationsthereof.

In other arrangements, fluorinated polymers other than PFPE can be usedto form alternating copolymers with PEO. Examples include, but are notlimited to, fluoropolyethers and perfluoropolyethers,poly(perfluoroalkyl acrylate), poly(perfluoroalkyl methacrylate),polytetrafluoroethylene, polychlorotrifluoroethylene, and polyvinylidenefluoride, and combinations thereof.

Examples of perfluoropolyethers include but are not limited to polymersthat include a segment such as a difluoromethylene oxide,tetrafluoroethylene oxide, hexafluoropropylene oxide,tetrafluoroethylene oxide-co-difluoromethylene oxide,hexafluoropropylene oxide-co-difluoromethylene oxide, or atetrafluoroethylene oxide-cohexafluoropropyleneoxide-co-difluoromethylene oxide segments and combinations thereof.

In one embodiment of the invention, alternating copolymers based on PFPEand PEO are combined with metal salts to form ionically-conductiveelectrolytes. Some useful metal salts are listed herein.

Formation of PFPE-Carbonate Alternating Copolymers

Scheme 3 below describes syntheses of PFPE-carbonate alternatingcopolymers using a simple polycondensation involving dimethyl carbonateand PFPE-diol. This reaction uses a catalytic amount of a base such asKOH or NaOH to activate the hydroxyl groups in PFPE. Values for z canrange from 1 to 10,000. By controlling the molecular weight of PFPE, theratio of PFPE to carbonate can be controlled, which in turn can be usedto tune the dielectric constant of the final material. Instead of usingdimethyl carbonate, phosgene (ClC(O)Cl) can be used to generate thecopolymer, however; excess base is used to scavenge HCl, which isliberated during the reaction.

In other arrangements, fluorinated polymers other than PFPE can be usedto form alternating copolymers with carbonate. Examples include, but arenot limited to, fluoropolyethers and perfluoropolyethers,poly(perfluoroalkyl acrylate), poly(perfluoroalkyl methacrylate),polytetrafluoroethylene, polychlorotrifluoroethylene, and polyvinylidenefluoride, and combinations thereof.

The fluorinated polymer segments may have molecular weights between 200and 400,000 Da or any range subsumed therein. PFPE-carbonate alternatingcopolymers may be solid, gels, or liquids depending on their molecularweights.

In one embodiment of the invention, alternating copolymers based on PFPEand carbonate are combined with metal salts to form ionically-conductiveelectrolytes. Some useful metal salts are listed herein.

Formation of PFPE-Carbonate-PEO Alternating Copolymers

Scheme 4 below describes syntheses of PFPE-carbonate-PEO alternatingcopolymers using a simple polycondensation involving dimethyl carbonate,PEO-diol, and PFPE-diol. Values for x can range from 5 to 10,000 and fory from 1 to 1,000.

By controlling the molecular weight of PFPE and the molecular weight ofPEO, the ratio of PFPE to conductive segments can be controlled, whichin turn can be used to tune the dielectric constant of the finalmaterial. Instead of using dimethyl carbonate, phosgene (ClC(O)Cl) canbe used to generate the copolymer, however; excess base is used toscavenge HCl, which is liberated during the reaction.

In other arrangements, fluorinated polymers other than PFPE can be usedto form alternating copolymers with carbonate and PEO. Examples include,but are not limited to, fluoropolyethers and perfluoropolyethers,poly(perfluoroalkyl acrylate), poly(perfluoroalkyl methacrylate),polytetrafluoroethylene, polychlorotrifluoroethylene, and polyvinylidenefluoride, and combinations thereof.

The PEO may have a molecular weight between 200 and 400,000 Da or anyrange subsumed therein. The fluorinated polymer segments may havemolecular weights between 200 and 400,000 Da or any range subsumedtherein. PFPE-carbonate-PEO alternating copolymers may be solid, gels,or liquids depending on their molecular weights.

In one embodiment of the invention, alternating copolymers based onPFPE, PEO, and carbonate are combined with metal salts to formionically-conductive electrolytes. Some useful metal salts are listedbelow.

Ionic liquids have been demonstrated as a class of plasticizers thatincrease ionic conductivity of polymer electrolytes such as PEO. It hasbeen demonstrated that the ionic conductivity of PEO can be increased bythe addition of ionic liquid, with the increase being proportional tothe amount of ionic liquid added.

In one embodiment of the invention, when the alternating copolymersdescribed above are mixed with ionic liquids they have higher ionicconductivity at low temperatures as compared to the copolymers withoutionic liquid, as would be expected.

Fluoropolymers and Perfluoropolymers with Terminal Urethane Groups

In one embodiment of the invention, an electrolyte made fromfluoropolymers and/or perfluoropolymers is used in a lithium battery.Examples of fluoropolymers and perfluoropolymers include but are notlimited to poly(perfluoroalkyl acrylate), poly(perfluoroalkylmethacrylate), polytetrafluoroethylene, polychlorotrifluoroethylene, andpolyvinylidene fluoride, and copolymers thereof.

Examples of perfluoropolyethers include but are not limited to polymersthat include a segment such as a difluoromethylene oxide,tetrafluoroethylene oxide, hexafluoropropylene oxide,tetrafluoroethylene oxide-co-difluoromethylene oxide,hexafluoropropylene oxide-co-difluoromethylene oxide, or atetrafluoroethylene oxide-cohexafluoropropyleneoxide-co-difluoromethylene oxide segments and combinations thereof.

Perfluoropolyethers terminated with methoxycarbonyl (MC) groups havebeen reported as lithium ion electrolytes when formulated with lithiumbis(trifluoromethane)sulfonimide. Examples of these are shown below.

The methyl carbonate termini of these polymers enhance the solubility oflithium salt in the electrolyte when compared to the diol precursors.However, linear carbonate groups do not make an inherently good solventfor salts: as analogues, solvents such as dimethyl carbonate and diethylcarbonate have almost no ability to dissolve lithium salts. Therefore itis likely that other functional groups may provide better saltsolubilities and higher ionic conductivities.

Some new materials made from perfluoropolyethers terminated withstructures significantly different from the methyl carbonate group havebeen synthesized and have been found to provide higher ionicconductivities than comparable methyl carbonate-terminatedperfluoropolyethers. This represents a new class of compounds that canbe especially useful as lithium ion electrolytes.

This new class of compounds can be generalized as either of thefollowing chemical structures. The first is terminated by a urethane atboth ends. The second is terminated by a urethane group at only one end.

wherein x (0≦x≦1) is the mole fraction of difluoromethyleneoxy groups inthe perfluoropolyether, 1−x (0≦x≦1) is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether, n (1≦n≦50) isthe average total number of randomly codistributed difluoromethyleneoxyand tetrafluoroethyleneoxy groups in the perfluoropolyether, and X iseither H or F. R^(F) is a perfluorinated C1-C8 straight or branchedalkyl group.

In one arrangement, the R¹ and R² substituents are each chosenindependently from short chain straight C1-C4 alkyl, branched C1-C4alkyl, 2-methoxyethyl, 2-(2-methoxy)ethoxyethyl, or cyanoethyl. Inanother arrangement, R¹ and R² can be combined with the N in a C5-C8heterocycloalkyl group such as pyrrolidine, piperidine, morpholine, or4-methylpiperazine. In yet another arrangement, R¹ and/or R² ishydrogen.

The terminal group consisting of an oxygen-carbonyl-nitrogen link(O—C(═O)—N) is known as a carbamate or urethane group. This class ofcompounds may therefore be generally referred to as urethane-terminatedperfluoropolyethers. The urethane group differs from the methylcarbonate group by substitution of a nitrogen. The effect of a nitrogenatom as compared to an oxygen atom is more easily appreciated in smallmolecules, such as in the properties of an ester and amide. Methylacetate has a boiling point (57-58° C.), has modest miscibility withwater, and is a poor solvent for salts. N,N-Dimethylacetamide has a muchhigher boiling point (165° C.), is completely miscible with water, andis a good organic solvent for salts. The last property in particularreflects the advantage of using urethane groups over methyl carbonategroups to terminate perfluoropolyethers for use as electrolytes: saltsare more likely to be dissolved and mobile. It is also more likely forthe urethane-terminated perfluoropolyethers to dissolve appreciableamounts of lithium salts other than LiTFSI, which can be useful whenformulating electrolytes.

Salts that can be used in the embodiments of the invention include, butare not limited to, alkali metal salts such as lithium salts, sodiumsalts, potassium salts, and cesium salts. Examples of lithium saltsinclude, but are not limited to, LiPF₆, LiBF₄, Li(BOB), LiClO₄, LiBETI,and LiTCB. Concentration of alkali metal salts in the electrolytesdisclosed herein range from 5 to 55 wt %, 5 to 30 wt %, 10 to 20 wt %,or any range subsumed therein.

The linear carbonate group is not inherently strongly polar and itspresence does not enhance the solubility of salts, a property crucialfor electrolytes. Incorporation of other more polar groups, such as theurethane group, imparts a higher polarity and results in better saltsolubility. Polarity refers to a separation of electric charge leadingto a molecule or its chemical groups having an electric dipole ormultipole moment. Polar molecules interact through dipole-dipoleintermolecular forces and hydrogen bonds. Molecular polarity isdependent on the difference in electronegativity between atoms in acompound and the asymmetry of the compound's structure. Polarityunderlies a number of physical properties including surface tension,solubility, and melting and boiling-points. Polar groups can alsofacilitate dissociation of lithium salts in an electrolyte; the betterthe dissociation of lithium salts, the higher the ionic conductivity inthe electrolyte.

Urethane-terminated perfluoropolyether compounds maintain the advantagesof perfluoropolyethers as electrolytes that have been previously cited,including low flammability and vapor pressure (for safety andconvenience), low melting point (enabling use at low temperatures, evenbelow 0° C.), and electrochemical inertness over a wide voltage range(appropriate for use inside an electrochemical device).

In an exemplary embodiment, synthesis of a dimethylurethane-terminatedperfluoropolyether (shown in Scheme 5 below) involved the followingsteps: A solution of 1H,1H,8H,8H-octafluoro-3,6-dioxaoctane-1,8-diol(5.0 g) and dimethylcarbamoyl chloride (4.39 g) in diethyl ether (80 mL)was prepared in a septum-capped flask and chilled in cold water. Asolution of potassium tert-butoxide (4.58 g) in tetrahydrofuran (40 mL)was added by syringe over 5 minutes. The mixture was stirred for 16hours. Water (50 ml) was added and the organic layer was retained,washed with 40 mL of 1 M hydrochloric acid, dried over magnesiumsulfate, and filtered. Evaporation of solvents and volatiles to constantweight left 6.96 g of clear oil, identified by ¹H- and ¹⁹F-NMR methodsas the desired product.

In an exemplary embodiment, synthesis of anotherdimethylurethane-terminated polyfluoropolyether (Scheme 5a, not shown)used a procedure similar was to that for the synthesis in Scheme 5, with1H,1H,11H,11H-perfluoro-3,6,9-trioxaundecane-1,11-diol (5.0 g) as thestarting material and adjusting the amounts of other reagentsaccordingly. The product was isolated as a clear oil.

In an exemplary embodiment, synthesis of a methyl carbonate-terminatedpolyfluoropolyether (shown in Scheme 6 below) involved the followingsteps: A solution of 1H,1H,8H,8H-octafluoro-3,6-dioxaoctane-1,8-diol(10.0 g), trimethylamine (8.59 g) and diethyl ether (160 mL) wasprepared in a 500 mL flask and chilled in an ice-water bath for 15minutes. A solution of methyl chloroformate (7.71 g) in diethyl ether(40 mL) was added at 2 mL/min over 20 minutes with continuous stirringand chilling of the reaction flask. The ice bath was removed and thesolution was stirred for 16 hours. The reaction was then transferred toa separatory funnel and washed with 2×50 mL 1 M hydrochloric acid, 1×50mL distilled water, and 1×50 mL saturated aqueous sodium chloridesolution. The retained organic layer was dried over magnesium sulfateand filtered before solvent was removed by evaporation leaving a cloudyyellow oil. The oil was washed with 2×10 mL hexane, then re-dissolved in30 mL of 2:1 ethyl acetate/hexane and treated with 0.5 g of decolorizingcharcoal for 30 minutes. The charcoal was removed by centrifugation andfiltration and the solvent removed by evaporation leaving 12 g of aclear colorless oil, identified by ¹H and ¹⁹F-NMR methods as the desiredproduct.

In an exemplary embodiment, synthesis of another methylcarbonate-terminated polyfluoropolyether (Scheme 6a, not shown) used aprocedure similar was to that for the synthesis in Scheme 6, with1H,1H,11H,11H-perfluoro-3,6,9-trioxaundecane-1,11-diol (10.0 g) as thestarting material and adjusting the amounts of other reagentsaccordingly. The product was isolated as 11 g of a clear oil.

Electrolytes Made from Fluoropolymers and Perfluoropolymers withTerminal Urethane Groups

Electrolyte solutions were formed by dissolving 10 wt % lithiumbis(trifluoromethane)sulfonimide (LiTFSI) in the appropriate liquid. Theionic conductivities of the electrolytes were measured by constructingsymmetric coin cells with porous polyolefin separators soaked throughwith the LiTFSI solution and performing electrochemical impedancespectroscopy. The results are shown below in Table 4. Conductivityresults for methyl carbonate-terminated perfluoropolyethers are shownfor comparison.

TABLE 4 Ionic conductivities of perfluoropolyether-LiTFSI electrolytesElectrolyte (with 10 Conductivity at 40° C. Conductivity at 80° C. wt %LiTFSI) (S cm⁻¹) (S cm⁻¹) Product of Scheme 5 3.6 × 10⁻⁵ 1.1 × 10⁻⁴Product of Scheme5a 2.8 × 10⁻⁵ 8.8 × 10⁻⁵ Product of Scheme 6 9.0 × 10⁻⁶1.5 × 10⁻⁵ Product of Scheme 6a 5.6 × 10⁻⁶ 1.5 × 10⁻⁵

Thus, urethane-terminated electrolytes (from Schemes 5 and 5a) haveionic conductivities 4 to 10 times greater than those of the methylcarbonate-terminated electrolytes (from Schemes 6 and 6a) under similarconditions.

Fluoropolymers and Perfluoropolymers with Terminal Cyclic CarbonateGroups

Some additional new materials made from perfluoropolyethers terminatedwith structures significantly different from the methyl carbonate grouphave been synthesized and have been found to provide higher ionicconductivities than comparable methyl carbonate-terminatedperfluoropolyethers. This represents a new class of compounds that canbe especially useful as lithium ion electrolytes.

This new class of compounds can be generalized as either of thefollowing chemical structures, according to an embodiment of theinvention. The first is terminated by a cyclic carbonate group at bothends. The second is terminated by a cyclic carbonate group at one end.

wherein x (0≦x≦1) is the mole fraction of difluoromethyleneoxy groups inthe perfluoropolyether, 1−x (0≦x≦1) is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether, n (1≦n≦50) isthe average total number of randomly codistributed difluoromethyleneoxyand tetrafluoroethyleneoxy groups in the perfluoropolyether, and X iseither H or F. R^(F) is a perfluorinated C1-C8 straight or branchedalkyl group.

In one embodiment of the invention, such structures incorporate6-membered cyclic carbonate rings:

wherein x (0≦x≦1) is the mole fraction of difluoromethyleneoxy groups inthe perfluoropolyether, 1−x (0≦x≦1) is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether, n (1≦n≦50) isthe average total number of randomly codistributed difluoromethyleneoxyand tetrafluoroethyleneoxy groups in the perfluoropolyether, and X iseither H or F. R^(F) is a perfluorinated C1-C8 straight or branchedalkyl group.

The linear carbonate group is not inherently strongly polar and itspresence does not enhance the solubility of salts, a property crucialfor electrolytes. Incorporation of other more polar groups, such ascyclic carbonate groups, imparts a higher polarity and results in bettersalt solubility. Polarity refers to a separation of electric chargeleading to a molecule or its chemical groups having an electric dipoleor multipole moment. Polar molecules interact through dipole-dipoleintermolecular forces and hydrogen bonds. Molecular polarity isdependent on the difference in electronegativity between atoms in acompound and the asymmetry of the compound's structure. Polarityunderlies a number of physical properties including surface tension,solubility, and melting and boiling-points. Polar groups can alsofacilitate dissociation of lithium salts in an electrolyte; the betterthe dissociation of lithium salts, the higher the ionic conductivity inthe electrolyte.

The terminal cyclic carbonate groups are similar to those of the smallmolecule ethylene carbonates. Constraint of the carbonate group in acyclic ring has a dramatic effect on its properties, as can be seen whencomparing the physical properties of small molecule cyclic carbonates ascompared to acyclic carbonates:

The much higher boiling points and dielectric constants for the cycliccarbonates demonstrate that cyclization causes physical properties tochange significantly. The properties of the cyclic carbonate-terminatedperfluoropolyethers may then be expected to differ measurably from thoseof linear carbonate-terminated perfluoropolyethers, especially thoseproperties pertaining to the solubility and transport of dissolvedlithium salts.

Appending 5-membered cyclic carbonate termini onto perfluoropolyetherprecursors involves a particularly different methodology from thatreported for the synthesis of linear carbonate-terminatedperfluoropolyethers. In one arrangement, the method involves a two-stepprocedure of first reacting the alcoholic endgroups with epichlorohydrinto form an oxirane intermediate, followed by a halide-catalyzed reactionwith carbon dioxide to form the 5-membered cyclic carbonate (seeExamples). The suggested 6-membered cyclic carbonate termini areincorporated via an ester linkage, which is distinct from the carbonatefunctional group.

The cyclic carbonate-terminated perfluoropolyether compounds maintainthe advantages of using perfluoropolyethers as electrolytes that havebeen previously cited, including: low flammability and vapor pressure(for safety and convenience), low melting point (enabling use at lowtemperatures, even below 0° C.), and electrochemical inertness over awide voltage range (appropriate for use inside an electrochemicaldevice). But the carbonate-terminated perfluoropolyether compoundsreported herein have the added advantages of increased solubility andenhanced transport of dissolved lithium salts.

In an exemplary embodiment, synthesis of a cyclic carbonate-terminatedpolyfluoropolyther (shown in Scheme 7 below) involves the followingsteps: A solution of 1H,1H-nonafluoro-3,6-dioxaheptan-1-ol (28.2 g),epichlorohydrin (18.5 g) and tetrahydrofuran (30 mL) was prepared in a250 mL flask. At 20 minute intervals, four (4) portions of 1.5 gpowdered sodium hydroxide (6.0 g total) were added, with vigorousstirring. The mixture was then heated to reflux for 5 hours. It was thencooled and filtered before removal of solvent on a rotary evaporator.The residue was vacuum distilled to isolate 17 g of the intermediateoxirane (boiling point ˜65° C. at 1 torr). 10 g of the intermediateoxirane were charged to a 40 mL vial and purged with dry CO₂ gas (50mL/min) for 15 minutes. Tetrabutylammonium bromide (100 mg) was added,then the mixture was heated with continued CO₂ bubbling in a 125° C.oilbath for 16 h. The solution was then cooled and washed with hexane(2×10 mL) before vacuum drying to constant weight, leaving 10 g of clearoil, identified by ¹H and ¹⁹F-NMR methods as the desired productcontaining <1 wt % of residual tetrabutylammonium bromide.

In an exemplary embodiment, synthesis of another cycliccarbonate-terminated polyfluoropolyther (Scheme 7a, not shown) used aprocedure similar was to that for the synthesis in Scheme 7, with1H,1H-tridecafluoro-3,6,9-trioxadecan-1-ol as the starting material. Theproduct was isolated as a clear oil.

In an exemplary embodiment, synthesis of another cycliccarbonate-terminated polyfluoropolyther (Scheme 7b, not shown) used aprocedure similar was to that for the synthesis in Scheme 8, with1H,1H,11H,11H-perfluoro-3,6,9-trioxaundecane-1,11-diol (10.0 g) as thestarting material and adjusting the amounts of other reagentsaccordingly. The product was isolated as 11 g of a clear oil.

Electrolytes Made from Fluoropolymers and Perfluoropolymers withTerminal Cyclic Carbonate Groups

Electrolyte solutions were formed by dissolving 10 wt % lithiumbis(trifluoromethane)sulfonimide (LiTFSI) in the appropriate liquid. Theionic conductivities of the electrolytes were measured by constructingsymmetric coin cells with porous polyolefin separators soaked throughwith the LiTFSI solution and performing electronic impedancespectroscopy. The results are shown below in Table 4. Conductivityresults for methyl carbonate-terminated perfluoropolyethers Scheme 6 andScheme 6a are shown for comparison.

TABLE 4 Ionic conductivities of cyclic carbonate terminatedperfluoropolyether-LiTFSI electrolytes Electrolyte (with 10 wt %Conductivity at 80° C. LiTFSI) (S cm⁻¹) Product of Scheme 6 1.5 × 10⁻⁵Product of Scheme 6a 1.5 × 10⁻⁵ Product of Scheme 7 1.6 × 10⁻⁴ Productof Scheme 7a 8.8 × 10⁻⁵

Thus, the cyclic carbonate-terminated electrolytes (Schemes 7 and 7a)have ionic conductivities 6 to 10 times greater than those of themethoxycarbonyl-terminated electrolytes (Schemes 6 and 6a) under similarconditions.

PEO-Graft-Copolymers

In one embodiment of the invention, PEO has been modified to reduce itsmelting temperature, thus also suppressing its crystallizationtemperature, by grafting it with perfluoro functional groups. When sucha PEO grafted with perfluoro functional groups is combined with anelectrolyte salt, it can be used as an electrolyte with good ionicconductivity at lower temperatures than had been possible for PEO alone.It has been found that addition of an ionic liquid to such anelectrolyte can increase the ionic conductivity even more than would beexpected.

As the proportion of perfluoro functional groups increases, both theionic conductivity and the T_(m) of the PEO graft-copolymer electrolytedecreases. At the same time, the decreased T_(m) makes it possible touse the PEO copolymer as an electrolyte at lower temperatures withoutits crystallization and concomitant reduction in ionic conductivity.Through careful experimentation, an optimal proportion of perfluorofunctional groups to include in a PEO graft-copolymer may be determinedfor a particular application.

In one embodiment, a PEO based polymer contains randomly distributed,grafted fluorinated groups. The fluorinated groups may be one or more ofperfluoro alkanes (PFA), fluoropolyethers and perfluoropolyethers(PFPE), poly(perfluoroalkyl acrylate), poly(perfluoroalkylmethacrylate), polytetrafluoroethylene, polychlorotrifluoroethylene, andpolyvinylidene fluoride, and combinations thereof. Theperfluoropolyether may include a segment such as difluoromethyleneoxide, tetrafluoroethylene oxide, hexafluoropropylene oxide,tetrafluoroethylene oxide-co-difluoromethylene oxide,hexafluoropropylene oxide-co-difluoromethylene oxide, or atetrafluoroethylene oxide-cohexafluoropropyleneoxide-co-difluoromethylene oxide groups, or combinations thereof.

In one arrangement, the perfluoro functional groups make up between 1mol % and 30 mol % of the PEO graft copolymer. In another arrangement,the perfluoro functional groups make up between 1 mol % and 20 mol % ofthe PEO graft copolymer. In yet another arrangement, the perfluorofunctional groups make up between 2 mol % and 5 mol % of the PEO graftcopolymer.

In one arrangement, the perfluoro functional groups have a molecularweight ranging from 200 to 500 Da. In another arrangement, the perfluorofunctional groups have a molecular weight ranging from 500 to 10,000 Da.In yet another arrangement, the perfluoro functional groups have amolecular weight ranging from 10,000 to 100,000 Da. In yet anotherarrangement, the perfluoro functional groups have a molecular weightranging from 200 to 100,000 Da, or any range subsumed therein. In yetanother arrangement, the perfluoro functional groups have a molecularweight ranging from 200 to 10,000 Da.

PEO-Graft-PFPE Copolymers

In one embodiment of the invention, grafting of PFPE onto PEO wasaccomplished by nucleophilic substitution of PFPE-based alkoxide onchloromethyl groups in P(EO-r-EPCH) as shown below in (8) below. Theamount of PFPE was controlled by changing either the grafting density orthe molecular weight of the PFPE reactant.

wherein x and y represent the relative mole fractions of the two typesof monomers in the copolymer and can have values from 0 to 100%.

In one arrangement, the pure PEO component makes up between about 20 and99 mol %, or any range subsumed therein, of the product polymer shown in(8) above. In another arrangement, the PEO component makes up betweenabout 50 and 90 mol %, or any range subsumed therein, of the productpolymer shown in (8) above. In another arrangement, the PEO componentmakes up between about 90 and 97 mol %, or any range subsumed therein,of the product polymer shown in (8) above.

In one arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 10 and 50 KDa, or any range subsumed therein. Inanother arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 20 and 200 KDa, or any range subsumed therein. Inanother arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 200 and 8000 KDa, or any range subsumed therein.In one arrangement, the molecular weight of PFPE ranges between 0.2 and15 KDa, or any range subsumed therein. In another arrangement, themolecular weight of PFPE ranges between 100 and 500 KDa, or any rangesubsumed therein.

Other fluorinated grafting groups are also possible in the generalstructure shown in (1). Cyclic perfloro alkanes such asperfluoro(methylcyclohexane) and perfluoro(methylcyclopentane) andaromatic versions such as pentafluorophenoxide and2,3,5,6-tetrafluorophenol may be grafted onto the PEO based polymer.

In one embodiment of the invention, PEO-graft-PFPE copolymers arecombined with metal salts to form ionically-conductive electrolytes.Some useful metal salts are listed below.

PEO-Graft-PFA Copolymers

In various embodiments of the invention, grafting of PFA onto PEO wasaccomplished using one of two different synthetic approaches, which areshown below. The amount of PFA can be controlled by changing either thegrafting density or the molecular weight of the PFA reactant.

The first approach, shown as (9) below, involves nucleophilicsubstitution of perfluro alkoxide, which is generated from hydroxyl PFAusing a base such as sodium hydride (NaH) or sodium hydroxide (NaOH).The perfluro alkoxide is reacted with pendant chloromethyl groups inP(EO-r-EPCH) to form the grafted copolymer.

In a second approach, a thiol-ene reaction between perfluoro alkanethiol and poly(EO-r-AGE) in the presence of a photoinitiator and underUV irradiation can be used to produce the graft copolymer in highyields, as shown in (10) below. The polymer product in (10) may besynthesized with various amounts of the pendant PFA units ranging from 1to 30 mol %, or any range subsumed therein.

wherein x and y represent the relative mole fractions of the two typesof monomers in the copolymer and can have values from 0 to 100%.

In one arrangement, the pure PEO component makes up between about 20 and99 mol %, or any range subsumed therein, of the product polymer shown in(8) above. In another arrangement, the PEO component makes up betweenabout 50 and 90 mol %, or any range subsumed therein, of the productpolymer shown in (9) or (10) above. In another arrangement, the PEOcomponent makes up between about 90 and 97 mol %, or any range subsumedtherein, of the product polymer shown in (9) or (10) above.

In one arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 10 and 50 KDa, or any range subsumed therein. Inanother arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 20 and 200 KDa, or any range subsumed therein. Inanother arrangement, the molecular weight of starting P(EO-r-EPCH)polymer ranges between 200 and 8000 KDa, or any range subsumed therein.In one arrangement, the molecular weight of PFPE ranges between 0.2 and15 KDa, or any range subsumed therein. In another arrangement, themolecular weight of PFPE ranges between 100 and 500 KDa, or any rangesubsumed therein.

Other fluorinated grafting groups are also possible in the generalstructure shown in (9) or (10). Cyclic perfloro alkanes such asperfluoro(methylcyclohexane) and perfluoro(methylcyclopentane) andaromatic versions such as pentafluorophenoxide and2,3,5,6-tetrafluorophenol may be grafted onto the PEO based polymer.

In one embodiment of the invention, PEO-graft-PFA copolymers arecombined with metal salts to form ionically-conductive electrolytes.Some useful metal salts are listed below.

In an exemplary embodiment, the graft polymer product in (10) wassynthesized with various amounts, ranging from 1 to 30 mol %, of pendantPFA units. The graft polymer product (10) materials were characterizedusing ¹H NMR and GPC. A representative DSC trace of graft polymerproduct (10) with 2.3 mol % PFA repeat units is shown in FIG. 8. TheT_(m) of this graft copolymer is 41° C. as compared with 60° C. for PEO,suggesting a strong influence from the PFA grafted side chains in thecrystallization temperature of the PEO-based copolymer.

In both of the approaches described in (9) and (10) above, the number ofgrafted PFA units can be varied by changing the amount of reactivefunctional groups in the starting PEO copolymer, either the —CH₂Cl inP(EO-r-ECH) or the allyl groups in P(EO-r-AGE). The number of PFA unitscan be chosen to tune the T_(m) and the resulting ionic conductivity ofthe final material.

PEO Graft-Copolymer Additives

Plasticizing additives may be combined with the polymer electrolytes toincrease the ionic conductivity of the electrolytes. Essentially, anysmall molecule can function in this role, but it is desirable to useadditives that do not react at the electrodes over the voltage range andoperating temperature range of the battery cells. When the electrolytesare used without additives, the system is a dry solid polymerelectrolyte. When additives are used, the system is a gel electrolyte.Non-volatile plasticizers are preferred when high temperature operationis desired. Such non-volatile plasticizers may increase ionicconductivity so much that cell operating temperatures can be decreased.The increased ionic conductivity from the non-volatile plasticizer(s)may compensate for the decreased inherent ionic conductivity from thelower temperatures. But such non-volatile plasticizers may also be usedstably at high temperatures due to their non-volatility, even totemperatures as high as 100° C. or higher. Dry electrolyte systems canalso be used at high temperatures (100° C. or higher) due to theirinherent non-volatility.

Ionic liquids are a class of non-volatile plasticizers that have beendemonstrated to increase ionic conductivity of polymer electrolytes suchas PEO. The ionic conductivity of PEO can be increased by the additionof ionic liquid, with the increase being proportional to the amount ofionic liquid added.

In one embodiment of the invention, when PEO-graft-PFA copolymerelectrolytes are mixed with ionic liquids, they have higher ionicconductivities at low temperatures than the same copolymer electrolyteswithout ionic liquid, as would be expected. In an unforeseen discovery,the increase in the ionic conductivity of the mixture of thePEO-graft-PFA electrolyte and ionic liquids is greater than the increasein the ionic conductivity of the mixture of PEO homopolymer electrolytewith the same proportion of ionic liquid. Without wishing to be bound toany particular theory, it may be that the PEO-graft-PFA electrolyte hasa higher affinity for the ionic liquid than does the pure PEOelectrolyte. A higher affinity may result in better mixing includingincreased miscibility of the polymer with both the ionic liquid and themetal salts.

In an exemplary embodiment, graft polymer product (10) (with 2.3 mol %PFA repeat units) was mixed with a lithium salt (LiTFSI) and 30 wt %ionic liquid (1-methyl-3-propylimidazoliumbis(trifluoromethylsulfonyl)imide). FIG. 9 shows a DSC trace for thiselectrolyte mixture (shown as 1) and for a mixture of PEO homopolymerwith the same salt and ionic liquid in the same proportions (shown as2). The crystalline melting peak from the PEO mixture sample at about48° C. is evident, and there is no such peak for the graft polymerproduct (10) mixture, indicating that crystallization has beencompletely suppressed below the testing temperature window in themixture with graft polymer product (10).

FIG. 10 shows graphs of ionic conductivity as a function of inversetemperature for cells made with various electrolyte mixtures, accordingto an embodiment of the invention. The following four electrolytemixtures were used:

-   -   a) graft polymer product (10) (2.3 mol % PFA), 30 wt. % ionic        liquid (1-methyl-3-propylimidazolium        bis(trifluoromethylsulfonyl)imide) (MPITFSI), and LiTFSi (r=0.1        equivalent with respect to EO);    -   b) PEO, 30 wt. % ionic liquid (1-methyl-3-propylimidazolium        bis(trifluoromethylsulfonyl)imide) (MPITFSI), and LiTFSi (r=0.1        equivalent with respect to EO);    -   c) graft polymer product (10) (2.3 mol % PFA) and LiTFSi (r=0.1        equivalent with respect to EO);    -   d) PEO and LiTFSi (r=0.1 equivalent with respect to EO).

Curve (d) shows the well-known ionic conductivity behavior of PEOelectrolyte. At temperatures above T_(m), ionic conductivity is on theorder of 10⁻³ S/cm. Once PEO begins to crystallize around 60° C., theconductivity drops sharply and continues to drop to 10⁻⁵ around 25° C.Curve (c) in which the PEO graft-copolymer is used shows conductivityslightly less that of PEO alone (curve d) at 80° C. and decreasesgradually throughout the temperature range shown but maintains a higherconductivity than PEO (curve d) at temperatures below the T_(m) of PEO.

Curve (b) shows that PEO electrolyte combined with an ionic liquid hasconductivity slightly more that of PEO alone (curve d) at 80° C. anddecreases gradually throughout the temperature range shown. The reallysurprising result is shown by curve (a) for the PEO graft-copolymercombined with an ionic liquid. Ionic conductivity is about 10⁻² S/cm at80° C. and does not drop to 10⁻³ S/cm until temperatures around 35° C.This is a significant improvement in ionic conductivity for a PEO-basedelectrolyte and occurs over a wide range of temperatures.

Optimizing Energy Density and Specific Energy

To achieve high specific energy in lithium (or other alkali metal)batteries, the proportion of active components (anode and cathode) ismaximized and optimally balanced, while the proportion of the auxiliarycomponents (separator, electrolyte and current collectors) that cannotstore energy is minimized. An optimized anode may be a thin foil oflithium metal, as discussed above, which serves as both the anode andthe current collector, and requires no additional ionic conductionwithin the foil as the lithium ions exchange at its surface with lithiummetal. A lithium foil thick enough to be physically strong andmanufacturable typically provides a large excess of lithium compared tothe capacity of the cathode. Thus, taking full advantage of the capacityof the overly thick anode foil would require thicker, higher-capacitycathodes as well. However, the actual thickness of the cathode islimited in practice by the depth to which both electrons and lithiumions can reach. Unlike with the lithium foil planar anode, ions andelectrons in the cathode must traverse the entire thickness of thecathode. Very thick cathodes may contain regions of active material towhich lithium ions cannot diffuse on relevant time scales dictated bythe cell cycling rate, rendering such regions dead weight and reducinguseable specific capacity of the cathode and therefore specific energyand energy density of the cell.

One approach to overcome these difficulties is to increase the ionicconductivity in the PE electrolyte of the cathode by using a liquidelectrolyte. Liquid electrolytes based on mixtures of organic carbonatesolvents with lithium salts and traces of performance enhancers are theindustry standard in commodity-type lithium ion batteries. But thelong-term stability of such batteries is limited, with shelf lifetimesof two years or less; the lifetimes are further shortened if thebatteries are cycled aggressively.

As discussed above, block copolymer electrolytes can act as effective,durable separators between the anode and cathode, providing sufficientionic conductivity for rapid charging and discharging while maintaininga physically robust barrier to prevent growth of dendrites from theanode or other detrimental breakdown. Such block copolymer electrolytescan also act as a separator between a PE electrolyte and the anode,eliminating detrimental interactions. In order to prevent any PEelectrolyte from absorbing into the block copolymer electrolyte andtraveling to the anode, it would be useful if the PE electrolyte and theblock copolymer electrolyte were immiscible.

Another problem in electrochemical cells such as batteries ispolarization (low transference number) of ionic and electronic species,which can result in suboptimal capacity even at low charge/dischargerates or high IR losses at high charge/discharge rates. A high lithiumtransference number (near 1, on a scale of 0 to 1) indicates thatmovement of lithium ions is predominantly responsible for the observedionic conductivity, with little contribution from the counter-ion. Inthe context of battery operation, a high lithium transference numberindicates that very little polarization occurs, as the counter-ions donot move and accumulate into concentration gradients.

Through careful choice of liquid PE electrolytes in the cathode andimmiscible block copolymer separator electrolytes, high ionicconductivity in the cathode, little or no polarization (lithiumtransference number near 1) can be achieved resulting in high specificenergy lithium (or other alkali metal) battery cells.

The block copolymer electrolytes discussed above have separatedmicrophases of ion-conducting segments and non-conducting, structuralsegments often possessing polar and non-polar natures, respectively.With both polar and non-polar components, many organic solvents would belikely to swell one or both of the phases, either of which would lead tostructural weakening. For example, some organic carbonate electrolyteformulations are compatible with cathode active materials and could becandidates for PE electrolytes. However, such formulations tend to beabsorbed by portions of block copolymer electrolytes, leading toplasticization (softening), weakening, reaction with the lithium anode,and failure.

As an alternative, heavily fluorinated molecules are known to beimmiscible with both polar and non-polar organic phases, and would notcause swelling in the block copolymer electrolyte disclosed herein. Ithas been reported that lithium electrolytes based on fluorinatedpolyethers have very high lithium transference numbers when formulatedwith a lithium salt (e.g., LiTFSI). See, for example, Wong et al,“Nonflammable perfluoropolyether-based electrolytes for lithiumbatteries,” PNAS Mar. 5, 2014 vol. 111 no. 4 3327-3331. Such afluorinated liquid electrolyte, and derivations thereof, are excellentcatholytes for pairing with a block copolymer separator; the catholytehas sufficient ionic conductivity, causes no polarization and does notswell or weaken the separator. Fluorinated liquids of sufficientmolecular weight can also be reliably non-volatile and non-flammable.

In one embodiment of the invention, fluorinated liquid electrolytes inthe cathode contain one or more of perfluoropolyethers, mono- ordiol-terminated perfluoropolyethers, alkylcarbonate-terminatedperfluoropolyethers, poly(perfluoropolyether)acrylates orpoly(perfluoropolyether)methacrylates, orpoly(perfluoropolyether)glycidyl ethers. In one arrangement, themolecular weights of the fluorinated liquids range from 200 Da to 10,000Da. In one arrangement, the liquids based on polymerizedperfluoropolyether-acrylates, -methacrylates, and -glycidyl ethers arepolymerized or copolymerized with each other or with small amounts (<10wt %) of other acrylates, methacrylates, or glycidyl ether monomers.Such copolymerization can change material properties, such as surfacetension, viscosity, and adhesion. Polymers formed from these fluorinatedmonomers would also be immiscible with the block copolymers mentionedabove.

Fluorinated liquids can have very low surface tensions, which would leadto leaching and spreading of the liquid out of the cathode if thecathode is not properly sealed. In some arrangements, the fluorinatedliquid electrolyte in the cathode is gelled. The fluorinated liquid isabsorbed into a polymer matrix to form such a polymer gel electrolyte.The polymer matrix may also be fluorinated to ensure compatibility withthe fluorinated liquid. Possible examples include high molecular weight(>10,000 Dalton) perfluoropolyethers, poly(perfluoropolyether)acrylates,poly(perfluoropolyether)methacrylates, orpoly(perfluoropolyether)glycidyl ethers, as well as copolymers and blockcopolymers of these with non-fluorinated polymers.

In one arrangement, the fluorinated liquid electrolyte is crosslinked.Depending on whether the mechanism of ionic conduction is or is notdependent on long range motion of the electrolyte molecules,crosslinking may have very little effect on the overall ionicconductivity of the electrolyte. Crosslinking past a certain thresholdmay cause the liquid electrolyte to become an immobile gel.Multifunctional or telechelic variants of the fluorinated polymerslisted above are examples of crosslinkable electrolytes.

Certain organic molecule additives may be added to the fluorinatedelectrolyte to improve electrochemical stability of the cathode activematerial. Such molecules may be added in small enough amounts that theywould not adversely affect other parts of the cell if they were todiffuse out of the cathode. Compound classes commonly used as additivesinclude cyclic organic carbonates, cyclic acetals, organic phosphates,cyclic organic sulfates, and cyclic organic sulfonates.

The ionic conductance of each component of an electrochemical cell canbe determined. In general, conductance, G is given by:

$\begin{matrix}{{G = \frac{\sigma\; A}{l}},} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where σ is ionic conductivity, A is cross-sectional area, and l islength. Two conductances in series, G₁, G₂ have a total conductanceG_(tot) given by:

$\begin{matrix}{G_{tot} = \frac{G_{1}G_{2}}{G_{1} + G_{2}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

When the electrode assembly has a composite configuration, the ionicconductance can be calculated easily from Equation (1). When theelectrode assembly has a multiple layer configuration, the conductanceof each layer is found, and the total conductance is given by Equation(2). In one embodiment of the invention, the conductance of the negativeelectrode assembly and the conductance of the positive electrodeassembly have a difference of no more than 25%. In another embodiment ofthe invention, the conductance of the NE electrode assembly, theconductance of the PE electrode assembly, and the conductance of theseparator electrolyte are all within 25% of one another. Matchingconductance in this way can result in a cell with an optimized, minimalimpedance profile.

Several useful fluorinated liquids may be used as PE electrolytes in theembodiments of the invention. The following examples are meant to beillustrative and not restrictive.

In one exemplary embodiment, a perfluoropolyether is terminated withtrifluoromethoxy groups. Note that m, 1−m=mole fractions of repeat units(0<=m<=1), n=number of repeat units (2<=n<=100). This structure is usedto define the R_(F) abbreviated structure referenced in the subsequentstructures.

In another exemplary embodiment, mono- and diol-terminatedperfluoropolyethers can be used as PE electrolytes:F₃CO—R_(F)—O—CF₂CH₂OH, HOH₂CF₂CO—R_(F)—O—CF₂CH₂OH

In another exemplary embodiment, alkylcarbonate-terminatedperfluoropolyether; R═C1-C8 alkyl, C1-C8 branched alkyl, or C5-C8 cyclicalkyl can be used as PE electrolytes:

In other exemplary embodiments, poly(perfluoropolyether)acrylate (R′=H)or poly(perfluoropolyether)methacrylate (R′=Me); k=number of repeatunits (5<=k<=50) can be used as PE electrolytes:

In another exemplary embodiment, poly(perfluoropolyether)glycidyl ether;k=number of repeat units (5<=k<=50) can be used as PE electrolytes:

FIG. 11 shows a complex impedance plot for a single electrolyte system(x) and for a two electrolyte system (∘). The single electrolyte cellcontains only a (first) dry polymer electrolyte optimized for stabilityagainst the lithium metal anode film. The two electrolyte cell has thesame first dry polymer electrolyte and a second dry polymer electrolyteoptimized for low interfacial impedance against the composite cathode.As is well known to a person having ordinary skill in the art, the sizeof the kinetic arc in the plot reflects the total resistance of thesystem. One might anticipate that adding an additional interface (firstpolymer electrolyte/second polymer electrolyte interface) could addadditional resistance to the system. But, surprisingly, the twoelectrolyte system has lower total resistance, as indicated by thesmaller kinetic arc (∘), than does the single electrolyte system. Thereis a clear advantage in using multiple electrolytes optimized for theirfunctions in the cell.

FIG. 12A shows specific capacity data over 500 cycles for a cell thatcontains one dry polymer electrolyte, a lithium metal anode and alithium iron phosphate composite cathode. There is no measurablecapacity fade over the first 100 cycles. After 500 cycles the capacityfade is estimated to be about 5%.

FIG. 12B shows specific capacity data over 100 cycles for atwo-electrolyte cell. The cell contains both a first dry polymerelectrolyte optimized for stability against the lithium metal anode filmand a second dry polymer electrolyte optimized for conductivity in andover the composite cathode. Again, there is no measurable capacity fadeover the first 100 cycles, indicating that there are no adverse effectsfrom using the multi-layered electrolyte.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

We claim:
 1. An electrochemical cell, comprising: a negative electrodeconfigured to absorb and release alkali metal ions; a separator layercomprising a first block copolymer electrolyte and a first saltcomprising the alkali metal; and a positive electrode comprisingpositive electrode active material, binder, a fluorinated catholyte, asecond salt comprising the alkali metal, and, optionally, electronicallyconducting carbon; wherein the fluorinated catholyte comprises a mixtureof perfluoropolyethers, each perfluoropolyether having two terminalcyclic carbonate groups covalently coupled thereto; wherein thefluorinated catholyte is immiscible with the first block copolymerelectrolyte; and wherein the separator layer is disposed between thenegative electrode and the positive electrode and facilitates ioniccommunication therebetween.
 2. The cell of claim 1 wherein a first blockof the first block copolymer electrolyte is ionically conductive and isselected from the group consisting of polyethers, polyamines,polyimides, polyamides, alkyl carbonates, polynitriles, polysiloxanes,polyphosphazines, polyolefins, polydienes, and combinations thereof. 3.The cell of claim 2 wherein a second block of the first block copolymerelectrolyte is selected from the group consisting of polystyrene,hydrogenated polystyrene, polymethacrylate, poly(methyl methacrylate),polyvinylpyridine, polyvinylcyclohexane, polyimide, polyamide,polypropylene, polyolefins, poly(t-butyl vinyl ether), poly(cyclohexylmethacrylate), poly(cyclohexyl vinyl ether), poly(t-butyl vinyl ether),polyethylene, poly(phenylene oxide), poly(2,6-dimethyl-1,4-phenyleneoxide) (PXE), poly(phenylene sulfide), poly(phenylene sulfide sulfone),poly(phenylene sulfide ketone), poly(phenylene sulfide amide),polysulfone, fluorocarbons, polyvinylidene fluoride, and copolymers thatcontain styrene, methacrylate, and/or vinylpyridine.
 4. The cell ofclaim 1 wherein the fluorinated catholyte comprises: anionically-conductive alternating copolymer comprising: a plurality offluorinated polymer segments, the segments comprising a mixture ofperfluoropolyethers, each perfluoropolyether having two terminal cycliccarbonate groups covalently coupled thereto; and a plurality ofnon-fluorinated polymer segments.
 5. The cell of claim 4 wherein thenon-fluorinated segments comprise one or more of carbonate; PEO; PPO;carbonate and PEO; and amide and PEO.
 6. The cell of claim 5 wherein thePEO further comprises cross-linkable monomers selected from the groupconsisting of oxiranes with pendant epoxide groups, allyl groups,acrylate groups, methacrylate groups, and combinations thereof.
 7. Thecell of claim 1 wherein the fluorinated catholyte comprises afluorinated liquid.
 8. The cell of claim 7 wherein the fluorinatedliquid further comprises one or more additives selected from the groupconsisting of cyclic organic carbonates, cyclic acetals, organicphosphates, cyclic organic sulfates, and cyclic organic sulfonates. 9.The cell of claim 1 wherein the fluorinated catholyte comprises: aplurality of PEO molecules; and perfluoro functional groups grafted ontoat least a portion of the plurality of PEO molecules to form a graftcopolymer.
 10. The cell of claim 9 wherein the perfluoro functionalgroups are selected from the group consisting of PFPE, polyvinylenefluoride, polyvinylfluoride, polytetrafluoroethylene, PFA, cyclicperfloro alkanes, pentafluorophenoxide, 2,3,5,6-tetrafluorophenol, andcombinations thereof.
 11. The cell of claim 1 wherein the alkali metalcomprises lithium and the first salt and the second salt are eachselected independently from the group consisting of LiTFSI, LiPF₆,LiBF₄, LiClO₄, LiOTf, LiC(Tf)₃, LiBOB, and LiDFOB.
 12. The cell of claim1, wherein the fluorinated catholyte comprises a second block copolymerelectrolyte, comprising: a mixture of perfluoropolyethers that forms afirst block; and a second polymer that forms a second block, the secondpolymer having a modulus in excess of 1×10⁵ Pa at 25° C.; wherein aplurality of first blocks associate to form a first domain and aplurality of second blocks associate to form a second domain, andtogether, the first domain and the second domain form an orderednanostructure; and wherein the perfluoropolyethers each has two terminalcyclic carbonate groups covalently coupled thereto.
 13. The cell ofclaim 12 wherein the first block of the second block copolymercomprises: an ionically-conductive alternating copolymer comprising: aplurality of fluorinated polymer segments, the segments comprising amixture of perfluoropolyethers, each perfluoropolyether having twoterminal cyclic carbonate groups covalently coupled thereto; and aplurality of non-fluorinated polymer segments.
 14. The cell of claim 12wherein the first block of the second block copolymer comprises: aplurality of PEO molecules; and perfluoro functional groups grafted ontoat least a portion of the plurality of PEO molecules to form a graftcopolymer.
 15. The cell of claim 12 wherein the second block of thesecond block copolymer is selected from the group consisting ofpolystyrene, hydrogenated polystyrene, polymethacrylate, poly(methylmethacrylate), polyvinylpyridine, polyvinylcyclohexane, polyimide,polyamide, polypropylene, polyolefins, poly(t-butyl vinyl ether),poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(t-butyl vinyl ether), polyethylene, poly(phenylene oxide),poly(2,6-dimethyl-1,4-phenylene oxide) (PXE), poly(phenylene sulfide),poly(phenylene sulfide sulfone), poly(phenylene sulfide ketone),poly(phenylene sulfide amide), polysulfone, fluorocarbons,polyvinylidene fluoride, and copolymers that contain styrene,methacrylate, and/or vinylpyridine.
 16. The electrochemical cell ofclaim 1, wherein the perfluoropolyethers comprise:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether; subscript “1-x” is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether; x rangesbetween 0 and 1; subscript n is the average total number of randomlyco-distributed difluoromethyleneoxy and tetrafluoroethyleneoxy groups inthe perfluoropolyether, and n ranges between 1 and 50; and X is eitherhydrogen or fluorine.
 17. The electrochemical cell of claim 4, whereinthe perfluoropolyethers comprise:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether; subscript “1-x” is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether; x rangesbetween 0 and 1; subscript n is the average total number of randomlyco-distributed difluoromethyleneoxy and tetrafluoroethyleneoxy groups inthe perfluoropolyether, and n ranges between 1 and 50; and X is eitherhydrogen or fluorine.
 18. The electrochemical cell of claim 12, whereinthe perfluoropolyethers comprise:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether; subscript “1-x” is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether; x rangesbetween 0 and 1; subscript n is the average total number of randomlyco-distributed difluoromethyleneoxy and tetrafluoroethyleneoxy groups inthe perfluoropolyether, and n ranges between 1 and 50; and X is eitherhydrogen or fluorine.
 19. The electrochemical cell of claim 13, whereinthe perfluoropolyethers comprise:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether; subscript “1-x” is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether; x rangesbetween 0 and 1; subscript n is the average total number of randomlyco-distributed difluoromethyleneoxy and tetrafluoroethyleneoxy groups inthe perfluoropolyether, and n ranges between 1 and 50; and X is eitherhydrogen or fluorine.
 20. An electrochemical cell, comprising: alithium-containing metal negative electrode foil; a separator layercomprising a first block copolymer electrolyte and a first lithium salt;and a positive electrode comprising positive electrode active material,binder, optional electronically conducting particles, a fluorinatedcatholyte, and a second lithium salt; wherein the fluorinated catholytecomprises a mixture of perfluoropolyethers, each having two terminalcyclic carbonate groups covalently coupled thereto; wherein thefluorinated catholyte is immiscible with the first block copolymerelectrolyte; and wherein the separator layer is disposed between thenegative electrode and the positive electrode and facilitates ioniccommunication therebetween.
 21. The electrochemical cell of claim 20,wherein the perfluoropolyethers comprise:

wherein subscript x is the mole fraction of difluoromethyleneoxy groupsin the perfluoropolyether; subscript “1-x” is the mole fraction oftetrafluoroethyleneoxy groups in the perfluoropolyether; x rangesbetween 0 and 1; subscript n is the average total number of randomlyco-distributed difluoromethyleneoxy and tetrafluoroethyleneoxy groups inthe perfluoropolyether, and n ranges between 1 and 50; and X is eitherhydrogen or fluorine.